Buy something simple and already built. Go for a series with little accident damage (so not Stock Hatch), and try and find something with stable regulations so you're not kippered every other year needing to change the car.
I would suggest looking at
www.csccgb.co.uk - the club is very friendly, welcomes novices and has all the above with the added benefit that the cars don't lose their value. Bits are readily available, and most people in the club are on a budget in the same boat.
Ken Clarke is still selling his XR3i which would be an ideal starter car in Group 1 Touring Cars. His number is: 01926 484140
He doesn't prep my Mazda, but I do buy bits off him - he's a very very knowledgeable guy.
Stacy
p.s sponsors can spoil the enjoyment of the day too. It's not all a plus.