Well, technically, as a late 90's deal, you wouldn't be looking at a duramax...just the awful, underpowered, not very efficient...but very simple and reliable GM 6.5 diesel. It more or less goes without saying to avoid anything with this engine at all cost.

Really, when it comes to either the 12V cummins, 24v cummins, the 7.3 powerstroke, and the early duramax engines, you can't go wrong. Engines themselves are all very rock solid and reliable...duramax might get a slight edge in mpg, but the cummins were pretty good to depending if it's in a 2wd vs 4wd, and if you've got 3.54's vs 4.10's (being so efficient making torque, I'd only look for the 4.10's if I was looking to tow over 10,000lbs). The 7.3 isn't a world beater in fuel efficiency; it has a good edge in non-towing, and of course, blows away gasoline engines towing fuel efficiency.

All are simple to work on...but a big inline six is always the easiest.

The transmissions are all strong units, each being able to take moderate modifications of the engines before you begin to reach the breaking point. Of course, each trans needs good/correct service, especially the much maligned dodge trans.

Looks is your preference. I'd always really like the dodge and fords...both look tough just sitting there. During that era, the Dodge is the oldest of the three (unless you really do look at late 90's GM trucks, which again, with HD trucks, there is no reason...they were a relative non-player in the HD truck segment).

Myself personally, if I had the time and money, I'd probably look for the real old style dodge ram (89-93) with a cummins, extended cab, manual trans, and 4WD. The trucks were relatively light and configured as such, could pull down as much as mid 20's with conservative driving. Find a clean one (they'll still get 5-8G clean!), go through with tires/brakes/etc/etc to make like new again and enjoy.

Yeah Ugly beat me but I'd say avoid the GM trucks with the 6.5, and hurts me to say that being a GM fanboy, but the Powerstroke and the Cummins are both superior. Always been leery of Chrysler transmissions though.

I wouldn't call the 6.5L reliable. They are pretty bad on injection pumps, and the heads like to warp.