Some of the 6.2 diesels were quite frugal with fuel consumption. I've heard some guys claim 20+ mpg in the right circumstances. A 4x4 won't do as well as a 2 wheel drive though.

Parts for the 6.2's used to be relatively economical, by diesel standards. Unfortunately, the 6.2's will be more likely to break than some of the better engineered diesels.

The 6.2 seemed to be designed for light duty applications. I've heard that using them for towing and other heavy duty jobs will increase the odds of them giving trouble. I don't know how true this is though.

The 6.5 Diesel is an evolution of the 6.2 diesel. Some of the 6.5's had a turbo on them from the factory. I used to know a guy who was into the 6.2 / 6.5 engines. He swapped some heavier duty parts from the 6.5 onto the 6.2 to help the smaller engine live longer. I don't recall all of the details now.

The 1990 RC will be a TBI engine. Those have a reputation for being reasonably reliable, but not real powerful. If you can find a 92 or 93 magnum powered RC it will have more power.