That's the first year for the cummins in a dodge truck. It's a turbo'd, NON intercooled 5.9. If you don't plan to tow heavy, the lack of intercooler won't be a problem. If you do, you can put on one from a later truck. That truck has the bosch rotary pump, called a VE pump. (I guess that's the model of pump) If the truck is a manual, it'll have the getrag 360 transmission. Those are weak, but can withstand reasonable power increases if you overfill the transmission with oil by a quart. If it's an auto, it'll most likely be a 727. I can't remember if they put the 518 4 speed auto in them in '89, I think not. If it's an auto you may well find it has 3.07 gears! 727's aren't terribly durable behind modified cummins.
There are things you can do to a VE pumped motor to get more power, and some of them are free. You can also get injectors and different turbos, as well as only changing the exhaust turbine housing for more boost. I THINK they're not wastegated, but I'm not 100% sure.
Overall, they aren't near as mod-friendly as the later p-pumped 12v's (those are from '94 to early '98), but they do respond well to some mods, and they get excellent mileage if they aren't an automatic with a stock (too loose) torque converter. They'll freakin run forever if you halfway take care of them. If you want to go fast with a cummins, you're probably better off trying to get a p-pumped 12v. However, if the price is right enough, you could probably get it and drive it for a while, doing the free/low cost stuff. Then, you might sell it for more than you paid and buy a later one.
