But dodgefan brings up a very good point. You said you wanted a good reliable car, that wasn't like your GA. That BMW you have your heart set on isn't really the perfect choice. BMW's aren't very famous for their reliability. Prestige, yes. Wow factor, yes. Sportiness, yes. Reliablity, absolutely not. Your GA gave you a good means of transportation for a while, and that's great, but I don't think that the Bimmer will last you much longer. In a year or so from now (maybe longer depending on how you treat it), it may occupy the very spot your Grand Am is in right now, in just the same shape. And even if you do take it to a mechanic, service and parts are extremely expensive, just for the fact that it's a BMW. I also should mention that 80's and 90's BMW's are absolute death traps in an accident.
I would seriously go for a better used car. Shop around for cheaper things that you know are going to be reliable (A J-Body or shadow will not be reliable, trust me).
I realize this'll sound sorta loony coming from me (Suggesting a Ford and all), but if you want a sporty, reliable car, that is good on gasoline, I'd suggest getting one of the last generation Cougars (the little ZX2-based hatchbacks) I still see zillions of them on the road every day, and how many years has it been since they stopped making them? I know it isn't RWD, but they are supposedly reliable and very fuel efficient. this is just a suggestion though (first thing that popped into my head).
The RWD thing sort of limits you down. There isn't really much of anything that was made in the past fifteen years that was reliable, RWD, and economical. Is rear-drive really that important? For someone in your position, I'd really just be happy to have a good clean reliable car. If that be the case, look into a Ford Thunderbird SC (here I go with the Fords again), I can't imagine they'd be too great on gas though because they're large, heavy, and supercharged. The LX Thunderbirds have V8's, so you know to steer clear of them if fuel economy is a priority. However, the N/A V6 Thunderbirds could work. I'm not sure of the gas mileage on those things but I don't think they'd be better than any 4-cylinder, less power moving the same amount of weight a V8 could move easily and everything.
Anyway, I wouldn't worry too much about the Bimmer. There'll be other cars that can serve your purpose a lot better. I'd also ditch that RWD mandate, because it does severly limit your options. Pick up an auto trader and leaf through that a while.