I looked at several Novas back in 1995- I was looking for a mid-twenties foot boat with twin SBC's. They are good-looking boats, very heavy but also very seaworthy. I didn't find one in the condition I wanted and wound up with a similar style boat, the Formula 242LS.
Drives shouldn't be an issue, neither should the 500 hours (that is low hours for a well-taken care of SBC). Alphas seem to work quite well in these boats with twin installs since they share the heavy load, and I have never had a problem with them in my Formula.
Take a good look at the stringers in the engine compartment- a couple I looked at had some problems there, although they had been stored outside, and were in a slightly abused condition.