OK, I'll chime in here as I have an '89 944. First, it is a 2.7 car not a 2.5. The 2.7s and 3 liters (a crank change is what keeps it from a 3 liter) do not suffer as badly as the 2.5 in the spun bearing department on track. Something about a pressure zone created under the piston, or something like that. A Porsche engineer explained it to me once, but its been a while.
You can get more hp out of it. They make a chip for it which gives you a bit, something like 10hp if you believe the hype, and then there's the header and exhaust route which can also open it up a bit. If you do it all, it should be giving close to 180 to 190hp at the crank, up from 168 stock for the 2.7.
The best mod by far though is to simply replace the throttle cam. I can't begin to tell you what a difference this makes. The stock cam puts most of the movement at the bottom end of the throttle pedal, where the new cams give it a very linear movement. The difference is night and day. It's absolutely the best $25 I've ever spent on a car bar none.
It turns the car into one you want to drive everyday from simply an OK car.
Expense wise, you do have to change the timing belt and assorted other front end hardware every 30k miles, and clutches are expensive. Other stuff is surprisingly cheap, especially brake hardware. Shocks and struts are also very reasonable.
I don't disagree with ddavidv very often, but I am on this one. I've owned both E30s and 944s, and I like the 944s much better. The '89's are faster than all the E30s except for the M3, and its not much off that one. Mid range is vastly superior, and its far more relaxed for everyday cruising. It sounds better, and the seats are much better.
Now for the bad, if its abused, simply turn and run away. You can spend a ton of money repairing the damage done by other owners. They've cheap for a long time, and they are not cheap to keep. The math doesn't add up for someone with no money or a Honda mentality for car maintenance. The same can be said for the BMW too, but moreso with the Porsche. It's much cheaper to buy one that's been well cared for in the long run, no matter how tempting the price. The '89's are by far though the best of the breed, be it a turbo or non-turbo.
I paid very little for mine, but it had a stack of dealer records, and a blown clutch helped bring the figure down even more. Previously though, I went through dollar spending hell on a turbo model. In the end it was nice, but it took a LOT of money to get it there.