I have a '84 turbocharged 4x4... .
They rust out from just looking at road salt, and once the rust starts it's very difficult to get it to stop. Literally one season of winter driving can start the process. I know of a 2wd D-50 that was rust free for over 10 years until it was driven one winter in Nebraska... ... it's now in a scrap yard. :{
The engine has pretty good power, about like a typical 4 cylinder gas engine. Mileage varies in the mid-high 20's. 30's is possible on a long highway trip.
Fairly low geared, 75mph is about it. They cruise GREAT 60-65mph.
Keep the engine fairly stock as the head gaskets don't take to excess boost pressure very well. If you lose a head gasket or overheat the engine, plan on a head as well. They aren't too expensive to repair--the Mitsubishi engine has a wide range of applications and parts are readily available.
The clutch isn't super strong--but if it needs replacement they're easy to find, and relatively cheap compared to other import little pickup clutches.
Adding a different air intake system and low restriction exhaust helps perk them up a little bit. With no muffler it cackles badly and sounds rather pathetic, a glasspack sounds pretty good.
In stock form they put out very little smoke, don't sound all that much like a diesel, and run pretty good. Mine has ~30" tires on it & I have no desire for more power. That's what the big truck is for.
The rest of the truck is pretty reliable. They have a few quirks here & there but overall I like them.