Okay...I had no time to read all of the posts cuz I gotta take a s***...but I've been researching RX-7's and I'm in the process of getting a 3rd gen. If you want a 3rd gen RX-7, go for '94 and up. Much more realiable from there. When you get the 7, there are a few things you need to do, for example you'll need an aftermarket downpipe because of how much heat the twin turbos produce..etc. etc.
You also have to take into consideration the previous owner(s) and milage. It may have less than 100,000 mi on it, but how hard was it driven and was it properly warmed up and cooled down.
As for the SRT-4, I think it's ok. It has huge aftermarket/oem support from Dodge and it was designed to take heavy modificatioins. It is a fast car and has the amp you need, who cares who the majority of the drivers are.
The BMW is a really good choice. I personally have 3 of them and I've had no problems at all. Parts aren't as expensive as you think and they last a long time. My '96 325i can pull on my P5 anyday, and that's a stock bimmer compared to a somewhat modified P5. I'm kind of biased but BMW's are good, realiable cars.
I really don't know anything about mustangs so I can't say anything here.
My vote goes for the RX-7 of course!!