Pick your championship or events and thisll steer you towards the right club (joining the right car club -and national club too- is essential if you want to get invited to the right events (I wont advise on national clubs because we will all end up squabbling)). Theyll be able to advise you with regards to the RAC MSA, but youll need a National B Race which will require an ARDS course and possibly a medical. The best advice for the car is to have the essentials completed by a professional prep company - catch tanks, lockwiring, cut-outs, re-routing brake and fuel pipes etc. Youll need a front and rear diagonally braced cage which can be a weld or bolt-in type (an can exceed the minimum spec usually, but it depends on the regs for your championship). Weld in is better but itll cost a lot more in labour. Kevin fits SD cages and whilst cheaper cages are available (I have one, in fact) - they arent anything like as good though. Ask Kev and he will happily describe the Rollcentre products shortcomings: and Id agree with him fully. Theyre legal though and they offer an FIA tube diameter option if you want. With regards to this, you need to decide before you start whether youll ever want to partake in any 'International' events, because they have different requirements to 'National' events. As well as the cage tubing diameter, an FIA spec seat and extinguisher system would be required, amongst other things.
If youre only going to race in National events then youll still need an approved helmet with a fireproof lining, but comically; you still dont need Nomex underwear, socks, lined shoes or gloves and you can race wearing what is effectively an EU approved welders boilersuit (looks like a race suit but the EN approval is for fireproof workwear!). I think that the helmet requirement is pointless, but Id always wear the best three layer Nomex suit along with underwear and balaclava that I could afford, irrespective of the minimum requirements.
Simon