Cliff
I do have a Formula 5000, a 1969 McLaren M10 A, chassis # 07.
I agree with most of the comments said above... race cars are race cars, and they are expensive to run, you have to pay constant attention to them, rebuilds.... if you want to play safe.
On the other hand, I think I would feel better if I have to repair or rebuild a modern replica after track days and accidents, than if I have to rebuild one of the only 17 chassis built by McLaren/Trojan in 1969.
And rebuilding the replica would be cheaper for sure than rebuilding any of those historic formula cars.
As Dave stated above there are plenty of possibilities these days out there to get a sexy car at a very good price. On top of that, running costs for those vehicles will have nothing to do with the expense of running any racing car.
Hope this helps
Cheers!
JP