Class 7 and class 8 semis for the quite a few years have been electronically controlled. I have 14 years experience supervising and managing in semi truck repair shops so I may be able to offer some advice.
The electronics are every bit as integral to the engine running as on the power strokes you are familiar with. A single sensor failure can shut you down just like ours. With that said, however, failures are not that common just like on our pickups.
A few questions, though.
Are you looking to get a full semi with a sleeper like THIS
or a day cab something like THIS ONE
? The sleepers can be converted to an RV and licensed as such. What you have to put in the sleeper to qualify will depend on what state you live in, I think.
There are three common engines. Cummins, Detroit Diesel and Caterpillar. Mercedes also makes truck engines and Mack and Volvo are making their own as well. There are several players in the class 7 trucks but these are the more common brands. In general you can get most of the engines in most any truck, with some restrictions (you won't find a Mack engine in a Peterbuilt from the factory, for example).
If the sleeper I would look for a Cummins N-14 engine. In my experience it is common for them to run well over a million miles without an inframe so if you can find one with 750 or so chances are it will last you a long time.
If the class 7 day cab (second picture) again a Cummins N-14 or Cummins M-11. Same longevity information applies.
The Series 60 Detroit Diesels seemed to need inframed a lot more often than the Cummins, usually around 650-700,000 miles.
I would stay away from the V-6 and V-8 Detroit Diesel engines in older trucks. They are relatively cheap to rebuild but needed it much more often than the newer engines. I would also avoid Caterpillar engines. They are reliable and such but parts are less available and more expensive.