youll want to check out some gear ratio calculators for your max speeds. its just one big math equation involving the the engine RPM, Transmission gear ratio, and the rear axle ratio which gives you your road speed. you are really only limited by the engine rpm. my truck does its best at 62mph on flat ground empty i can achieve 15mpg at about 2200 RPM. now the truck physically WILL go 75, but the engine is turning like 2800 rpm...so it will go 75 BUT its probably getting about 5mpg, wouldnt take too long to overheat it and its being beaten to death. maybe if you are using it just for personal use a few times a year that may not be a problem, but for a business fuel economy and maintenance can really put you out of business. but anyway, check out some truck gear ratio calculators. these are pretty accurate as long as you put in accurate numbers. the biggest being the tire size. generally ive found most of these trucks to have the standard 11R22.5 (the "tall 22.5") better yet is the lo-pro 22.5, like 275/80 R22.5. smaller tire will turn faster giving you a little bit more road speed at the same rpm, OR a lower RPM at the same speed. so going to a low pro can either gain you a bit of speed or drop your rpm. when looking at the calculators, keep in mind that every engine is different, mainly the different class of engine. medium duties (6-10 liter engines) are higher rpm motors, mine for example is 190HP @ 2300 rpm/ 485 ft lbs. @ 1500 RPM, where as the larger engine (11-15 liters) are lower rpm motors, for example the 60 series detroit 12.7L is 450hp @ 2100RPM and 1500 ft lbs at 1200RPM. generally an RPM somewhere between the rated HP and Torque number will yield the best power, fuel economy, and reliability. so 2000 RPM on the medium duties and 1600 RPM on the heavy duties is what you would be looking for.
for the calculator numbers you can find tire diameter on most manufactures or tire dealers websites. some calculators use the TIRE RPM (Tire Revolutions Per Mile) - also located on tire manufactures websites, or the Tire Diameter, also on tire websites. the diameter of my 275/80 R22.5's is about 39-1/2". the other number you will need rear end ratio, and transmission gear ratio. for these trucks, as we talked about, a typical rear end is 4.44, and 4.11, i have seen 5.63's (mine) and a couple others but the 4.44 and 4.10 are pretty common. again a gear swap is cheap if done your self, and these parts a extremely plentiful in truck salvage yards. when you are ready to buy a truck, the VIN number can be used to determine the rear gear ratio by doing some internet research, or even calling a dealer and giving them the vin to check. on a side note for clarification, the small bellhousing is SAE2 and the big trucks use SAE1 (i always mix them up lol). for transmission gearing, again the VIN can be used as stated above. in person, you can find the rear axle tag and the tranmission tag to verify what they are. there are a TON of different big truck transmissions, theres like 50 different versions of eatons 13 speed, all which are different gear sets. typically the model number is what you look at. "OverDrive is typically in the 0.80's (0.83, 0.85, 0.87) and a Double Over Drive is the the 0.70's (0.73,0.74, 0.75) so you have Direct Drive transmissions, which the highest gear is a 1.00. an Overdrive in the 0.80's, and the double over in the 0.70's. the double overs will give you a faster road speed and a lower RPM but they are less fuel efficient. heres a link for eatons 13's Eaton Fuller 13 Speed Transmission Ratios - Global Transmission Supply
the model number is what to look for. an RT is a direct drive, and RTO is an overdrive, and an RTLO is a double over. with a direct drive tranny you would need really really high gears (2.60's) so you would want a single over if you are going to use 3.55/3.73 and a double over for anything other than that is you wanna do 70-75mph.
all this plays in to how well the truck will drive, as well as the fuel economy. heres a little story to explain why speed inst always everything...
i started driving truck as an regional flatbed driver, i was in a 95 freightliner FLD120 with a Cat 355HP and a 10speed with 3.55 rears. it was governed at 68mph and i was always loaded to gross at 80k lbs. it did fine on the highway, usually had to drop only one or two gears going up hills on the interstate and still hung in there around 60mph. after that i ran a local dump truck...an old 80's International with a big cam cummins and 13speed. man was that thing SLOW! it was an old municipal plow truck, had crazy low gears. it would top out at about 55mph wide open, did ok at 45mph but it took a beating to go 55. it really drove the same as my Freightliner up until it would max the road speed out. it went thru all the gears pretty fast and got up to 45 about the same as my FL, but it just couldnt go any faster. shortly about that the owner retired that truck do to maintenance reasons. he replaced it with a newer International, a 1990 9400 eagle with a 450HP 3406 Cat...with an 8LL behind it.... WOW. all that Horsepower makes you think its going to be GREAT, right? wrong... that damn 8LL was HORRIBLE!!! ya the truck would go 65mph no problem, when it was empty it was fine. but when you load it... it was just as slow as the old dump truck and my 80,000 pound freightliner would out run it on the hills all day long! this is the problem with spec'ing the RIGHT combination of transmissions and rear end gears. as soon as you would come to even the slightest hill you have to drop a gear. the 8 speed doesn't have all that many gears and the gap between them was pretty big. so even tho you only had to drop 1 gear it took your road speed from 65 down to 55.... loaded you would have to drop 2 gears so now you are doing 45.... it was the worst engine/trans/rear gearing combination in a truck that i have ever driven!! id put money on my current truck - 190hp DT466e and 7speed loaded to 36k would probably out run that 450hp cat dump truck with the 8speed, not only that but it just sucked - like it wasn't any fun driving that thing - all that power and it couldn't go up a hill 45mph lol. moral of the story - SPEC THE TRUCK RIGHT!
heres the spicer 7 speed book, page 3 has the gear ratios.
look at the weight ratings and the gear ratios of the different versions, another reason to look at that when you are looking to do things "outside the box" with these trucks, as these medium duties where meant to be a TRUCK not a tractor - and typically weren't configured for anything other than TRUCK WEIGHT, where a tractor is meant to have a trailer and typically gross 80-100k with out any modifications.
in our case here with medium duty straight trucks and trailers. i can tell you that mine WILL be getting a 10speed (my favorite) or a 13 speed. at 36k the dt466e is more than adequate, again providing you aren't looking to break land speed records or outrun the truck beside you up a hill. the biggest problem is the trans. the spicer 6+1 (7speed) is a great tranny, BUT it is a double overdrive only. so it goes from 1.00 in 6th gear to 0.74 in 7th gear - it doesn't have the single overdrive between 1.00 and 0.74. the 9-18 speeds DO, so in a 13speed - 11th would be 1.00, 12th would be 0.84 and 13th would be 0.74 - much like that over powered-under geared dump truck i was talking about. when you come to a hill and need to drop a gear in the spicer 7 speed you are actually dropping 2 gears double over down to direct. so when you gotta drop a gear you lose a LOT of road speed.
in your case i would buy a truck well under your budget, these trucks are so common and so cheap that you can get a good one at a super cheap price and still have a lot left over for mods. im thinking your best bet would be (CDL wise) is to buy a 33k single axle truck that already the right length for your application (24' body) as a frame stretch is going to run you 5k. dont forget to look at the cab and chassis trucks and box trucks too, most places that sell bodies will remove an existing and install a new one - used bodies can be had for 3k and if you buy a box truck you can trade in your box on the price of the flat. use your budget to do a tranny swap, buy a good used 13speed and an SAE 2 bellhousing for it and its a direct swap other than running air lines to the shifter and probably the drive shaft length.
the more gears the better - it will allow you to stay in the power band more often (better fuel economy and a better driving experience) some of the 33k trucks did come with a 9 speed, you can check the specs on that specific tranny and then see what the numbers would be in the calculator.
using the calculator, my truck with 5.63's would have the following speeds/rpms:
Spicer 7 speed
7th - 2200RPM - 62MPH
6th - 2200RPM - 46MPH
5th - 2200RPM - 34MPH
Eaton 13speed RTLO
13th - 2200RPM - 63MPH
12th - 2200RPM - 53MPH
11th - 2200RPM - 46MPH
you can see how much less road speed drop there is when you need to drop gears just haveing the ONE extra gear in there. 16MPH loss in the spicer when you drop from 7 to 6 where as there is only a 10mph drop as the same rpm when you drop from 13th to 12th.
now lets try that with the 4.10 rear end gear ratio AND dropping the cruising RPM from 2200 RPM DOWN TO 2000RPM
Spicer 7 speed
7th - 2000RPM - 77MPH
6th - 2000RPM - 57MPH
5th - 2000RPM - 43MPH
Eaton 13speed RTLO
13th - 2000RPM - 78MPH
12th - 2000RPM - 67MPH
11th - 2000RPM - 57MPH
so ya, theres some good numbers for you - the spicer would let you go 75mph with 4.10's but like i said - you wont be going very fast when it comes to hills of any kind. where as a 13speed would be WAY better for drive-ability and fuel economy.