If you want to just get out there then probably not that much you need to do. My view on the minimum would be:
Roll Bar, Extinguisher, Electrical cut off, Harnesses, decent brake discs/pads, Nitrons or similar, Nylotron bushes, Uprated toe links, Full Geo and a test day to fine tune, Tyres (whatever the regs allow). Also you will need to race prep the car, going through everything to check nothing is worn - ball joints, wheels bearing, tie rods etc.
I’ve considered doing a round just for a giggle and that is the absolute minimum I would do to my car before attempting it. Also, remember there is always the very real possibility of stuffing your car, particularly if you are a novice.
I think that we met at Anglesey a few weeks back. There was certainly someone going mental in a bright orange Exige!
You can find a copy of the series technical regulations in pdf form here http://www.midenginedsportscars.com/series/
The absolute minimum you HAVE to have to race is like Randy said:- An electrical cut-off switch and an extinguisher of (I think) at least 2.25 litres capacity. Both these need to have an external pull cable or push button and be clearly marked. A Four point Harness. Front and rear tow points. Of course overalls, boots, gloves and a snell approved or BSA type A helmet (I had my ECE approved motorcycle helmet confiscated by the scrutineers at my first race).
At the moment we have to run on Yokohama moulded treaded tyres which really means no slicks or cut slicks. Everyone seems to run on 048s at the moment.
So come and join us! Have a laugh, have a fire! It doesn’t have to cost you the earth.
I hope that answers a few of your questions.
I think Azreal has answered your question. One other thing to realise is that you may start having thoughts about more mods, bigger brakes, etc etc, and then before you know it, the car is no longer road legal and your credit card is up to its limit.