Current (soon to be previous) gen Civic Si? Awesome gas mileage, trick six speed, decent handling, F1 like revs, huge aftermarket, room for four (if you decide you need that - if you don't, switch rec to S2000) and reliability for years to come. I wouldn't worry too much about the "ultimate track car" if you're just starting out. I think you'll find that, like any kind of racing, when you get out there, some particular brand/class will catch your eye and then you can buy the dedicated race car later and get exactly what you want. For example, I had never given GTI's a second look until I started autocrossing against them. Now I covet . . .
My advice would be buy a solid, low mile DD that's going to last forever (hence, Civic Si rec) but can occasionally be tracked. Get out there, figure out exactly what you want, then buy a dedicated track car after bumming some rides/co-drives at the track and narrowing down whether you want FWD, AWD, RWD, Spec series car, TT car, etc., etc. There's just to many variables right now. You may even find its just something you want to do a couple times a year and doesn't justify the expense, work, tow vehicle and storage that comes with a dedicated race car. If you "split the baby" now, you're either going to have one car that's neither a good DD nor a good race car or you're going to have two cars, likely neither of which is exactly what you want.