I really enjoy my '89 FC. It started life as a GTU (base model), but I retrofitted the 4-pot brakes. A 4.30 clutch-type rear is on the list for next year.
Stance coilovers are a pretty good budget option. By the time you get Konis plus the GC kit you're over a grand without camber plates. Toss in a set of GC camber plates and you're up around $1300. I paid just under $1100 for my Stance setup with custom spring rates. They're very beefy and well made. The shock valving isn't perfect, but off-the-shelf Koni valving isn't perfect with high spring rates either.
If you pick up an FC you should join the Mazda Motorsports program (mazdamotorsports.com). It's free, and you'll save a ton of money on parts.
As for the car in general, the first priority is a full tuneup. Spark plugs, plug wires, O2 sensor, vacuum lines, belts, rad flush, thermostat, coolant hoses. Get the engine good and clean as well so that you can see if anything is leaking. Make sure your MOP (Metering Oil Pump) system is working. If unsure, you can premix a bit to ensure good lubrication of the apex seals (search 'premix' on rx7club.com for tons of reading on this subject).
If it hasn't been done already you should replace all the bushings. Suspension, engine mounts, trans mounts (including the the main 'saddle' mount), diff mounts, and perhaps even the rear subframe mounts if they're dicey. Go with urethane for the suspension, and at least the 40% stiffer Mazda competition drivetrain mounts. These cars are old, and the stock rubber will be crap by now. Put in the DTSS eliminator bushings in the rear. It's not hard with time and the right tools. You'll be amazed at how much better the car feels and responds with fresh bushings all around.
Ongoing, ALWAYS be aware of your engine temps. Heat will kill a rotary dead in no time flat. Also, pay attention to your 2nd gear synchro. This is a weak spot in FC trannies. I've had good luck with NEO Synth gear oil, but just had to have mine rebuilt with 95k miles on it (it has been used hard all of it's life).
Power-wise, the first thing to do is to check out the main cat. If it's original, chances are that it's starting to get fouled. Mine was pretty clogged by 90k. Magnaflow sells a reasonable replacement cat w/downpipe that replaces everything between the stock manifold and the y-pipe. It runs about $140 and is a good compromise if you can't afford the Racing Beat header + replacement cat. If you live in a state that doesn't do emissions testing, look into the Racing Beat true-dual exhaust. Good power gains there. For about $400 you can get an RTek modified ECU. It has programmable fuel and timing maps in the stock ECU case and can help dial in your tune for best power. Your gas mileage will improve too!
Good luck!