Jack,
The value is subjective, how much are you willing to pay ?
You can always raise your offer, I have no idea what HE expects to get or what his bottom line.
Dont be afraid to insult him, based on your assessment it is a good car to restore but needs work, I would offer him $ 7,500 and go from there, others here will beg to differ with me perhaps.
What is a # 2 car worth, $ 15 or 20 K ?
Youll spend $ 7.5K on all the work you mentioned, including new paint easy. If you have floor rust expect to find lower qtr. panel rust, etc. IMO.
Good luck,
Porter