CSP would be where you play. Honestly, you could probably run STX locally, and nobody should really complain too much(ask your competitors, and see if they are OK with it first). the only 3 things that really take you out of ST and make you SP are the boost, and the DP, and the IC.
If your locals don't want you in STX, I'd probably run SMF, as the PAX index is easier for you( CSP- 0.860 vs SMF - 0.848) so you'd be doing a little better overall with the same time in SMF than you would in CSP. (ah yes, index racing is fun, isn't it... lol)
Overall, just go out and have fun, meet cool people, learn your car, and decide if you want to dive deeper into modding, or if less is more.