if not mx5 then maybe get a mk2 golf and put the engine from mk 4 into it...
Unfortunately, if you start with that attitude, you will find it very easy to build a car that isnt eligable to run in any class that you are likely to be competitive in.... Sounds harsh I know, but a lot of people build what they fancy and then find that they are put in with the big money guys if they are eligable for any class at all and imediately and understandably get put right off in a very short period of time...
As above, check out the ARDS website to see how much a race license will set you back following the medical, license test and general fees for licenses, followed by the MSA website for a look through the blue book of regulations (and wild confusion normally!) and then take a look at the Go Motorsport website for a guide to some "cheap" forms of motorsport. As long as you bear in mind that MOTORSPORT IS NOT CHEAP, you will be fine but I must repeat, MOTORSPORT IS NOT CHEAP! FWIW, both the MX5 and the Golf GTI championships are VERY EXPENSIVE to get into and to be even remotely competitive.
So thats the negativity dealt with, seriously, motorsport kicks ass! If you can afford it, its a serious buzz turning up to your first event (even if you get pasted its still a buzz!) and once you figure out your competitive, it gets like all good drugs, ADDICTIVE!!!!