The main venue is SCCA club and national racing. I race a D sports racer w/ the SCCA Washington DC region and find the organization to excellent. SCCA, relative to other sanctioning bodies is more bound to tradition and rules - the GCR (General Competition Rules) is the holy writ and it's running close to 800 pages now. The classing structure is huge, and crazy, though if you're keen to race a Pinto, a bug-eye Sprite, or anything without fenders they're your people.
As for F500s. I see them every race weekend. In the MARRS series they're run with the formula vee cars generally, although at the final round this season the 500s were tossed into the car salad that is the "Wings and Things" group. For comparative laptimes, I was doing 1'18's down to 1'17"s in the 15 minute qualifying session which was 11 laps for me. I started P1, and almost lapped one of the 500s. The other was faster.
If you have wheel to wheel experience and are a good mechanic/tuner/light fabricator, ~and~ the car is a very good deal it may be worthwhile. But bear in mind that an unknown race car that's been sitting can hid large unpleasant secrets. For example, I learned that the fuel system in my (purchased amazingly cheaply) Radical, which worked fine at Summit Point, couldn't keep up on the much larger, faster full circuit at VIR. This was discovered at the Grassroots Motorsports Ultimate Track Car Challenge and cost me 600 miles of towing, entry fees, hotel, and abusing a fresh set of Hoosiers driving around off-line, in the infield, and doing laps of the perimeter road between sessions trying to diagnose the problem.
I'd add up the cost of the car plus the cost of freshening the engine, refastening the chassis w/ new hardware, new harnesses, re-certifying the fire bottle, verifying that the fuel cell foam hasn't turned to crud, and the value of your time to sort the car out to then point where you're able to concentrate on driving this atomic death toboggan rather than thrashing to make it to the grid. If this number is enough lower than what you'd be willing to buy an F500 for at the prevailing prices, go for it.
I ask myself a qualifying question when deals like this pop up too; "Would I make a deliberate choice to go race a whatever if this perceived great deal wasn't in front of me?"
In most cases it's cheaper to buy a well sorted race car that's ready to race, particularly at the end of the season from someone who's motivated to become an ex-racer. Set up notes, spares kit, everything turn key. There's value there.
One thing to consider if you don't mind a project is the move afoot for F600. Lose the unobtainium 2 stroke motor and install a 600cc sportbike engine. Seems like a great idea to me as a way to make what's a bit of a dinosaur class have more modern relevance. Sort of like what they're doing w/ F1000 or FB, which is a 1000cc bike engine in a formula continental, and formula fit, w/ a formula Ford converted with a Honda Fit engine that comes from Honda Motorsports.
A useful place to look is:
F500 people geek out here.
