The '90's Chevy 6.5L diesel was very underpowered and not that reliable. The '01 - '04.5 LB7 Duramax has problematic injectors. If you want a Chevy diesel I would stay with a '04.5 LLY or newer.
The Ford 7.3 was a great engine but they normally command high prices even today. The 6.0 is very problematic and should be avoided unless you are prepared to spend thousands of dollars fixing all the problems with them. Also, resale will be quite low. The Ford 6.4 had a reputation as a fuel hog and is even more expensive to fix than the 6.0.
The Cummins 5.9 was pretty much solid no matter what. The problem with them was the poor 4 speed automatic transmission that so many were equipped with. If you can find a 5.9L with a manual it would be best.