decide on a budget, decide on what you want from it, i.e. track day thrasher with no road use is cheapest way of building one, basic road use or a fully kitted out show standard car as the cost will vary massivly. once you've decided on that get along to a show or 2 and have a look around a few companies and have a sit in some cars as the size also varies a lot between kits.

dealing with a company you can drive to would be a bonus as with a lot of kit companies response time on phone calls, emails etc is pretty slow and it can be a real ball ache if you're waiting for 1 part holding up your build.

i personally have a MK INDY, it's a fairly old kit now, chassis was built in 2003 but the quality is good, the panels are a decent finish, i had good dealings with the guys at MK and it didn't cost the earth to build.
if i was to do it again now i'd probably go for a MNR as they seem to have some nice, well thought out cars and a good reputation.