I'm trying to get something like this off the ground around here.
The key is less about costs (since a well painted body costs more than the whole darn car) but focus on who is the better driver not who has the better setup - and as you say if you don't allow any adjustments to be made you limit the class getting out of control. (I personally would allow cosmetic mods, just so people can feel they have something different, even though they have no performance impact).
Trick is trying to get the ones that have 4-5 xrays to drive a tub car again. It's not like they are budget sensitive

Seth