We've actually had quite a few of those. I think they're the most fun, and people get really creative with it as well. Good numbers to work with are somewhere between 100,000 and 150,000 credits. That way you can either get a cheap car and fully load it, or get an expensive car and put good tires and shocks on it.
Keep in mind, you need to make it all inclusive. Purchasing the car, all the parts, oil change, everything. And anyone who turns in a competetive time needs to report all the money they spent in an itemized list so that other members can check to see if he / she is cheating.
Best of luck! This is one of my favorite competitions, when I have time for them.
