The downside on the RB20 is you won't find one in the states that wasn't already imported by someone else or you'll have to import your own. The bonus is that it was being built by Nissan during the same time they were building the S12 so it fits their style of mounts from that time.

Glad my advice helped. Personally I'm very into specifically Miatas but I know Nissans alright for someone that's never actually had experience on them. I just pick it up from people like Indecisive who tell me all about it. Really, that guy might as well have helped develop that car. He's the easiest to get to and the best resource you could possibly find for these cars.