Hello mate,
I am completely new to this aswell as I joined at Christmas but haven't managed to get down for any racing yet. Found it hard to find time to build my buggy in between work, entertaining kids and a newborn just arrived. Lol.
Probably should have taken your route and bought a rtr model. As far as 12v battery, I think any old car battery will do but if I am wrong I am sure that one of the other members will put you (and me) straight. I will probably be using the battery from my caravan (its a bit big but should save me a few quid in not having to fork out on another battery).
As far as tyres go, I was told to get schumacher mini spikes green for the wet and yellows for the dry. I think some people use silvers for the wet too. Kev at inside line racing is really helpful and as a Stotfold member you get 5% off all orders and he can bring them to the club which will save on postage. Sure keith or adrian will be able to give you the discount code.
Don't take this the wrong way but hopefully you and your son will be just as bad as me when we're down there as it will make me feel a lot better if I have someone equally as crap. Lol.
A lot of the guys down there do seem very helpful and friendly though. Hope to see you soon fella.
Gary