The SB-800 is the king of compatibility if you can find one at a good price - works with everything from standard film TTL, through early digital D-TTL, to current i-TTL, and also has non-TTL auto and manual modes. Earlier flashes won't do i-TTL, while recent ones won't do film TTL or D-TTL. The lower power SB-600 does most of this, but I think drops non-TTL auto.
The F-5 (and every other Nikon I’ve checked up on) supports up to 250v for sync voltage.
Prices for SB-800’s seemed to have dropped a bit from when I bought mine last year, so it may be a good time to buy one. As mentioned, they are fully compatible with every film body that supports TTL, with a slight exception of the F3, and there is a flash shoe adapter available for the F-3 that makes it compatible too.
Yes, the F5 was the last pro film Nikon camera that was used by professionals. By the time the F6 made the market the industry was already shifting to digital.
People mentioned lithiums, yeah, they are the way to go for the F5. I couldn't give exact numbers of rolls per battery set as there are many variables but F5 battery life is excellent.
It'll always be an iconic and historical camera. There are many around currently and prices are incredibly cheap but in time this is the type of camera the price may really jump.
I have one of the first F5's to ever ship worldwide so it's quite unique and I could never sell it.