I'm with those who say find out which classes are most popular where you'll be racing; in our region (DC) ITA and ITB are the top classes. From your list you can't go wrong with an ITA Miata, but the Integra also does very well in that class. The CRX is a great car for either ITA or ITB. If I were racing ITS I would probably go with the RX7, but the Miata and Integra both can compete well there. Or, if I liked to wrench on cars I might even try to find an old 240Z to play with.

Now, with that said, if I were going to start an IT project today I would look at ITR; no, there isn't a huge amount of competition right now, but I think few years from now this class will have some of the best racing in IT.