rx7's are not as rare as you think. Series 6,7 were sold here in australia and cost a fair bit compared to imported serie 8 rx7's. There are heaps of them in the country now and can easily be imported if you cant find what you want here in australia. The ones you would be mainly thinking of are the series 6,7 that can only be imported for track cars as they were sold here by mazda.
I would say get the rx7. You wil have a lot more fun in a rx7 than a s15. They are reliable when looked after and maintained properly ie warmed up and cooled down properly. They are awesome cars to drive and modify and can make big horse power for small engines. And since you wont be using it as a daily driver you wont have to worry so much about fuel.