I'd recommend an already prepared or a semi-prepared E30 with good records. They are quick, reliable and predictable.

I would not recommend an E36, not a for a beginner. Wayyyyyy to fast, like shock/springs/tune/gutted/R comps NOTHING else 1:40 on Road Atlanta fast and you've invested $5000. Also, their extremely aggressive rear suspension geometry really requires someone with experience. If you think you're up on that level, then by all means get one..

Miata's are always good choices, just make sure you invest in safety equipment. By the time you can run a fast lap time in one you'll probably want a cage