TBH very few street cars make excellent track cars straight out of the box. some will be transformed with the addition of a few tweaks (WRX's respond brilliantly to rear ARB) while others are just plain crap, no matter how much you throw at them.

Personally I think that the Rex has more potential to go fast than the FN, though it is probably flogged and will need some TLC to get it up to scratch. When new the 99-00 WRX's were very sweet and balanced (first time I drove my friend's 99 model with 165,000km on the clock was enough to convince me to buy a Rex)

An EP3 will make for a more fun road car than the FN, and be quicker around a track, but you'll still be slow compared to a lot of dedicated track cars. It will also need mods to be properly quick.

If you want a reliable car for the road, and a quick track car, you will be hard pressed to beat an Evo 6/7. You can still get a reliable one under $20k, and it doesn't have to be heavily modded to go quickly. The WRX's aren't too bad, either, but tend to be a bit more fragile than Evos. I've had plenty of success at track days, motorkhanas and dirt sprints in various bog standard Rexes (as well as the Forester XT family hack), so don't let the AWD thing throw you. Don't get too hung up on using the handbrake either. Even in wet motorkhanas I never bothered with the handbrake and managed a second outright in the Forester (pipped by a tarmac rally Subaru Legacy, who was doing a shakedown to compete in Targa NZ at the time)

There are currently some bargains for AWD gravel rally cars, which can easily be transformed to tarmac spec. You'll have the bonus of having a cage and harnesses. Of course these won't be suitable for daily duties, but I see Evorexes crop up for low teens quite regularly, so you can afford a daily as well as dedicated track (gravel) toy.