I like the whole progression idea, but it would be tad more realistic (if that's the right word to use) if only the budget increased, as it would add a bit of strategy as to whether or not you kept a lower class car that you've tuned up (i.e one of the fastest Class C cars) versus starting afresh with a new car (one of the slower Class B cars).
So perhaps everyone would start off with $25,000, and everyone would get another $10,000 each week to spend on their car or save towards getting a faster car. You'd also need an adjudicator/book-keeper to keep track of people's finances, but you could do that pretty easily using a shared Google Doc or something.
Progressing via Class makes it more gamey imo, not necessarily a bad thing, but that's up to Clarence or whoever started this thread :p
Anyway, just blue-sky thinking ;)