If the only issue is the finish, it'd be very hard to go wrong with an older 870 at that price. Even harder if you can talk him down a bit. My only concern, actually, would be the restoration. What exactly does that mean, and did a decent gunsmith do the work? If I just dropped in at a pawn shop and spotted a "restored" 870, I'd be a little hesitant, because I wouldn't know if it was done by someone competent. However, if you took your 642 in to the shop for work, I'm guessing that you trust the smith. If someone I trusted did the work, I'd jump on it at that price, assuming I had the cash to spare.
From time to time, I duck hunt with one that was probably made in the 50s or 60s. It's been used to paddle a boat, submerged in water, and spackled in mud while bouncing along on the back of a 4-wheeler. It still shoots like a champ.
Edited to add: I wouldn't worry too much about the box or the manual, if this is going to be a shooter. With a gun that old, I wouldn't expect the box to be around (unless it's a collector's piece, of which I have none), and I'd bet that you can download the manual off of Remington's site.
I'm a lawyer, but I'm not your lawyer. If you need some honest-to-goodness legal advice, go buy some.