The easiest engine to build, period, is my avatar.
I'm tellin' ya', a 5-year-old could build one of those.
Both the 6.9L and 7.3L diesel engines used in Ford full-sized pick-ups were made by International. In fact, they're pretty much the same engine, the 7.3 simply being the later, larger displacement version of the 6.9 (granted there are a few other differences, but they're very minor).
As for your question, on a more practical
level, probably one of the older factory models, before everyone in the world fell in love with electronics and computers.
Check out one of the diesel powered mini-trucks from the 80's.
In fact, I believe the engine Nissan used back then was ubiquitous "4-33".
That was probably the single most used engine back in the 70's, during the "diesel conversion" craze.
But really, any of the older applications. Even if its an old Mercedes (and apparently, a recent google search has turned up quite a "240D" cult happening these days).
Things to check for, when you go car/truck shopping is where are the glow plugs and are they really
that easy to get at, because affer filters, they're going to be the second most common "replacment part" on your diesel engine....and remember, ALWAYS use Bosch glow plugs (don't learn the hard way, like I did).
Ok so I am new and not new to diesel's, I went to vocational school for diesel technology, while I was only there for a short time, I really didn't get much into the diesel stuff, because the teacher made things more difficult then they needed to be so I left that class and went to an automotive class.
As I've read on various diesel forums, Its recommend to work on a 12V cummins due to it being a mechanical engine, but as I read more and more, I read that the 7.3l IDI and 6.9 are also mechanical engines. Now don't give me wrong I have nothing against dodge I just prefer ford powerstroke etc. I know these aren't Powerstroke engines, but isn't the 7.3L the first built international diesel engine built by them, because IIRC the 6.9 was a ford built diesel?
On a side note, I have a decent knowledge of diagnostics especially when it comes to computers so that's what used to and not mechanical wise. Its far much easier to play with a OBD II reader etc as opposed to just guessing with a mechanical engine.
I honestly don't know what to get in all honest, changing the injectors on a 7.3L you gotta pull the cab off IIRC right as well as the 6.0. I've read of the issues with the 6.0 but if properly tuned and maintanenced theres little to no issues. maybe someone on here can point me in the right direction. I have though about going back to school for it, but really don't have the patience for it, and I kinda get more out of reading on the forums anyway.