As mentioned already, if you want to allow racers to turn up with their cars and use their existing transponders, you need an AMB (Mylaps) system. These are expensive new, and even used ones fetch £1000+. If you have hire cars and need transponders these are around £70 each.
A lot of newer/smaller clubs are now using the Kamtec IRace system. This is only £300 with extra transponders £12. It's an infrared system and won't be quite as accurate as an AMB, but it's fine for indoor club racing and I know of lots of clubs using it happily:
http://www.kamtec.co.uk/IRACE-Lap-Sc...d-complete-set
You'll then need some software to go with it. You can get a free trial of RC-Timing which will do you 10 full race meetings/events, after that it's a £345 purchase:
http://www.rc-timing.com/