Well, the Porsche and M3 are probably out of the question due to just the buy in costs of the vehicles. Not including any modifications.
What kinda track stuff you plan on doing? Autox? Road Racing with clubs?
Can't go wrong with a Mx-5 Mazda... throw on a Jackson racing supercharger and some suspension mods. And you've got a car that'll do anything... AutoX, Road Race, Drag, Drift... whatever!
And still be streetable.
Mmmm S/C'd Miata... or swap in a 13b turbo motor, put on a hardtop and call it a baby FD Rx-7.