100/4 only went up to 56, so it's not 56-57
If it's a 100S give them 3000 (£ $ anything else), get it on a trailer, get it locked up safely ready for auctioning at a vast profit. (I think it is VERY unlikely that it is, only 100 were made and I think they are all accounted for) It could be a 100M, but there were factory 100M and aftermarket ones, so you need to know which in order to decide on a price.
The bodies are (at least partly) ali, but the thing to worry about is the chassis and underframe which you can't see, and rust like heck!
3000 sounds like a price for a car that needs a complete rebuild, not for something you're going to polish, change the oil and plugs and drive.