Trevor put a good list together. Brakes and wiring would be up on the top of the list of things to go through. These are robust little cars, and not hard to work on at all. Font suspension will probably need to be looked at, but if it's functioning you may be able to get a year or 2 of driving before that project.
The drums, if in good shape, are well able to stop the car from the speeds you'll get from a 948. In the flats it will move the car around well, but not quickly (factory spec was 0 - 60 in something like 30 seconds). It will feel faster than it is. 50 in a Bugeye feels very fast.
Ray - I had wondered about the coil on the generator, thinking that was from an earlier build date. Don't have my book handy, so couldn't verify. Also, I think it was your car I was thinking about with those quilted covers. Never thought I'd see 2 with that "improvement."
Depending on the rust situation, $6500 is a decent asking price, giving you room to negotiate down for issues you find on inspection (such as the things mentioned here, and any rust issues). The modifications don't seem too out of place, but they do hurt the value a bit. For a driver, they're not significant since you may change/update things anyways.
On the other hand, there are plenty of Bugeyes around, so don't settle for less than you really want. There's something to be said for an unmodified car, even if you plan on modifying it yourself. At least that way, you know for sure what you have and can always go back to stock at a future date.