Along with a price offer perhaps offer him rides in it and progress updates as you make any.
This is a real tough call. You want to offer enough to get the truck at a reasonable price to you, but still be fair just as you stated.
Since he bought the truck new and knows the original cost he may not expect as much as he knows the truck is actually worth. He may also be able to provide you with some original paperwork which to some Stovebolters is worth a lot extra.
Tough call, but best wishes for both being happy.
