I think the convertible idea is not so smart because a fiberglass hardtop is not cheap, and there's really no advantage over a coupe. Also, why buy an RSA when you are going to change the suspension anyway? if you go 964 just buy a cheap C2.
Don - this page should answer your questions on the 911 cup rules. no 964s. I think this is a very good idea and looks relatively easy to build to (or buy to) without spending tons of money:
http://www.911cup.com/classrules.html
but, have you driven a kart? you can rent them for cheap in your area, most of the tracks at this link rent karts, just call. worth a (very fun) try for $100 or less, right?:
http://www.na-motorsports.com/Tracks/KartTracks.html#OutdoorCA