The gs-t is front wheel drive, the gsx is all wheel. Some options are more commonly found on the gsx. The gsx is rare because of supply and demand. Not as many were sold, and they're the most coveted. AWD is an advantage. And people like to own the "best" of everything. They'll run well as long as they're taken care of, no magic number when things slide downhill. The year to year changes were few, in assuming you're looking for a second gen, at least. 95's have eprom based ecu's, which is nice. Early 95's have vented rotors. 99's are the most expensive.
The most common problem is disregarding maintenance. Not replacing the timing belt is bad, for example. You'll hear talk of crankwalk, it's not common. A new turbo is much cheaper on a dsm than on a Prelude. A turbo that will get you 350whp will cost 500 dollars new. You still need supporting mods, like fuel support and tuning.
Mpg, not so hot. There's a discussion about this right now on this page of the board. On the turbo models, not much more than 20. Top speed, no idea. The gst is electonically limited to 130ish. That can be removed. Weight is 3200 pounds. Hp is 210, torque is about the same. Stock, slower than a vr-4. Keep in mind, you're asking questions about stock performance when no one drives these cars stock
For three thosand dollars, plus the cost of the gsx, you could run 12's.
You're asking good questions, but many of them can be answered via the faq that says "read before posting." One of the links in there in particular, 1000aaq, may help you out a bit. Good luck.