...question is...how much do you want to have in it when it's finished...?
Sitting for so long you could easily drop a grand or two getting it back up to snuff, that providing there's nothing actually wrong
with the bike.
BMW Rubber and plasticky parts are expensive.
If you don't wrench add a grand to have a dealer get it basically maintained right.
I'd offer $2500 thereabouts.
But what a jewel it might be!
...hmmm...does he have the 3/4 solo seat...