As has been said, it is the class the club runs and gives you people to help and races to be in. If you have the choice, then 2WD Off-Road by buying an RTR. I bought an AE B4 RTR car and added some springs and 30deg castor blocks (a tenner the lot as I remember) and was competitive.
If you find GT12 at your local club, then you need a car, the Core package, some red front springs, Contact Tyres from the box with 37 rears and 50 fronts on standby and an Ascari shell. Build it and then race it. Like all pan-car classes it is what you are able to do with your thumbs that counts, not the depth of your pocket.
People will say GT12 is not a cheap class, but that comes from those who think they can buy speed and handling - you can't. In that respect it is the cheapest class, but only when you realise that getting driving skills is the key to being fast, and not the depth of your pocket.
If I could find a club running it, I would say 2WD Off-Rod is the best place to start. The RTR cars are very good, and the cars are more tolerant of set-ups. Driving skills still count, but you will need the right tyres for each track and you will probably need some new springs. HTH
