For an early SS machine, I usually look at $500 for a non working project, up to $1500 for a fair to good example in working order and up to $3500 for a showroom mint example.
By the sound of it, this FP is in working order but needs some detailing and TLC applied so I would say 900-1200 would be a fair price. It all depends on closer inspection of backglass, playfield, cabinet, etc. Probably also a good idea to play for no less than half hour and check for any gremlins.
FP is one of my top 5 favourites and I don't have one so you can't go wrong in owning one. Sounds like an interesting find.