Hi Marc, just to answer a couple of your questions, the entry level classes are really class 1 and class 2. Class 1 would be the equivalent of production cars in your day, unmodified up to 1000cc, it used to be just Minis but now includes 106s, Saxo and Micras. Class 2 allows slight modifications on the engine and suspension, Novas, Corsas and Micras are most popular. The build regs for both cars are pretty much the same. As Ron says come to the AGM, have a chat, decide which class you want to do, apply for a licence then you will receive a rule book which will have all the regs in it to build a car. If you go onto the NASA website which is linked from our link page you will find more information on the other classes available which are all very modified.
As an honest answer from a race car builder, you would be best to go second hand for your first season in the sport, new cars can break the bank just to get on track as there is always unseen costs, class 1 can now be done for a decent budget thanks to the nissan micra being allowed in, but class 2 allows a little more flexibility when it comes to things you're allowed to do and tinker with