To hit the major points. It must have a trans cooler and either have been rebuilt or had oil+filter changes every 20-40K miles. If its missing one of those things, except to rebuild that trans at some point. Otherwise, they are great transmissions, they are even great when their trashed and need to be rebuilt. Mines needed a rebuild for over a year, it doesn't like 2nd gear, and its strange to shift from 1st to 3rd in an automatic, but I do it every day, and I have to be careful with the throttle or it'll slip out of gear, but it still gets me where I need to go from 10mph to 80mph. Its pulled 12ft enclosed trailers full of furniture at 60mph without a problem.
The engines are rock solid, they'll outlast the body and frame if you do regular oil changes. The oil pump grenaded in my old engine and it still pulled that trailer I mentioned 30 miles at 60mph back into town without any oil. It was still running the next day when I started pulling it out.
If it's 4wd people beat the **** out of it, its a given. The transfer cases are pretty tough, but the front axle vacuum disconnect isn't. The most common problem is loss of vacuum which means no 4wd, but those actuators like to break at about this age. The rear end is pretty solid, not a lot to worry about there.
The only real electrical issues you'll have with it, is the light switch, those like to burn out, and front lighting issues if you have the sport model and try to put anything but halogen bulbs in.
Also, the lenses like to fog up real bad.
Gas mileage will make you cry, my old 5.2 2wd would get 18mpg highway if I took all my crap out and it was bare truck, 14-15 city. Since I put the 5.9 in it, I get about 14-15 highway and 10 city. 4wd trucks can knock a number or two off those. A lot of 4wd guys with lifts and big tires get 12-13 no matter what they do.
A tune will change the way it drives, won't fix the gas mileage or turn it into a racer, but it'll give it a nice boost.
These aren't Chevys or Fords, they won't get great fuel economy like Chevy, and they won't have the maintenance free lifestyle of a Ford. But man are they reliable trucks, I've yet to see the day my truck doesn't start on the first two cranks, even in the -5 degree weather we get in the winter. Neither Chevy nor Ford can pull as much weight as our dodges down the highway. I fully loaded my truck down with a 7Klbs trailer plus the 5Klbs my truck weighed at the time and I still pulled it with my 5.2 down the highway real easy and got about 11mpg doing it. My buddies Chevy dropped from 18-19 to 8 pulling a similar weight, and his dads ford dropped to 5 pulling slightly less weight.
I love my Dodge, I would never trade it for anything but another Dodge, even then I still wouldn't, not after all the blood i've put into this truck. I love it so much i'll write a small book about them for someone who MIGHT buy one. Thats the kind of trucks these are.