Don't waste your money with the KO system, it's unreliable.
If you wnet AMB, make the guys buy the personal transponders. More and more tracks are going that route. Not having to fool with handout transponders for most tracks helps cut some overhead costs.
But with just a few guys having fun in a parking lot, big time AMB systems are probably out of their reach. In that case find someone with an old laptop and printer and then download a copy of a race program called LAPSFREE. It will keep track of the race via manual input from someone at the computer punching in the car numbers as they pass a start/finish line or whatever point you decide. It's easy to use and with 4-5 cars, one person can easily keep track of the race, then print it out at the end. Having an extra person calling out car numbers as they pass and standing beside the computer person helps.
You can find the software at
www.RCTiming.com And best of all, it's FREE!
Good luck and have fun!