I'm currently competing in 1A in my NA6, but my only direct competition is a pair of Pulsars. I would gratefully welcome another NA6 in 1A! I support the principle of starting with a slow car and learning to drive it quickly, and for this there's no better place to start than with an NA6!

In 1B, you'd be very competitive in an NC and do well in an NB too. Until a decent driver in an S2000 or Elise decides to run.
In 2A, Jason O'Meara is blitzing the field with his Civic. Try 1:09.999 at Wakefield Park and 1:51.273 at Eastern Creek. He's a very good steerer. Second is still up for grabs though. NA6s are still competitive and with the right mods and plenty of practice I reckon you could eventually get close to these times.
I'd say forget about 2B if you want to win. Rex's Lotus is damned quick and he has a pair of S2000s in close pursuit. We have a few guys in NBs and NCs running midfield in 2B and a fair way behind. They're all quick drivers but it's a tough ask.
If you're just getting started I'd suggest limiting yourself to either type 1 or 2. MX-5 or something else? One factor that might pursuade you towards an MX-5 is the strong camaraderie that we MX-5ers share. We regularly have about 15 drivers at each round, and it's a great social experience. You won't get that with any other make/model (unless you want to get a Clubman - they're strong too).