The value in purchasing a previously built/raced car lies in the number of reliable, appropriate, and costly components that have already been purchased and installed. Additionally, the quality of the build and the level of "Sorting" that has already occurred can be valuable. Would recommend you consider the following...
Does it have upgraded suspension
Does it have upgraded braking - lines, rotors, calipers
Does it have a legal seat and belts
Does it have an appropriate fuel cell, pump, and lines
Does it have a Lemons compliant roll cage
Does it have an appropriate and compliant kill switch
Does it come with appropriate wheels and at least two sets
Has the cooling system been upgraded to include larger radiator, shroud, fans
What is the REAL condition of the motor. compression and leak-down data
What is the condition of the transmission and shift linkage. how many races on the tranny
If the car does not meet most of the above criteria, regardless of whether it is Lemons, SCCA, or circle track, then you would be better off continuing to look until you find one that does meet the above. When you do find one...it will result in significant overall savings versus a new build. If you can't find something that meets most of the criteria, you might as well start from scratch, as it can often take more time and money to resolve somebody else's poor build than to just start from scratch.
And don't worry about the $500 car issue w pre-owned car.
#9 Audi CQ Scooby Doo Car known as the Mystery Machine, #44 Audi 4KQ painted as a tribute car to the 1989 IMSA car driven by Haywood, Stuck, and Rohrl, and the #95 Audi TT Lightning McQueen.