Hi JB,
Welcome aboard.
I agree with Howard that racing is all about development. Just ask Ross or Iain!
However I disagree with Howard that you want the car with the best suspension geometry, that can be relatively easily changed. Again see Ross and Iains threads.
Imho you need the lightest car you can get that has a soundly designed basic chassis that you can work with and modify to suit your own requirements. Doing this is a helluva lot easier than trying to take weight out of a fat pig that will otherwise need more horse power, bigger rubber, bigger brakes just to try and match a lighter less powerful car.
Mono or spaceframe? One more for the hornets nest! Spaceframe of course! But if you're a 'pay-somebody-to-work-on- it' sort of guy it doesn't matter coz you can just have a spare tub sitting in the corner of the workshop. But if you're going to fix it yourself and cost is an issue, then spaceframe for sure.
Be sure to check your regs to see what you can and can't do, that could also have a major bearing on your decision.
There's heaps of discussion on here about all things. It's all been covered before! Frinstance, do a search on "transaxle" and see what you get! LOL
Then you can try "mono" or "spaceframe". After that go for "suspension". Or whatever takes your fancy! That should keep you quiet for the rest of the year!
Good luck with your research.
Cheers,