Actually, finding parts for the rx7 is MUCH easier than the ae86. If you want something just hop on ausrotor or ns.com or the various mazda workshops out there. It'll be there. A good healthy Fc won't kill you on fuel, you should be able to get it at around 12-13k lter per 100km. It's no 4age economy, but then again, with 2-3 times the power you wouldn't expect it to be. And if you do a lot of freeway driving, the rx7 is actually very very good on fuel.
And when you're drifting you want as much grip as possible. More grip means you can go much faster and have way more control over the car. The FC also has a 50/50 weight distribution and pretty short wheel base which alows for fast transitions