E-bodies aren't fetching what they were. Yes, it'll be more valuable than your A-body now, or if the prices ever get back to where they were, but, really, in my opinion, if he's asking in the $20's for a car that needs everything, especially a 318 car, then he's out of line with today's market.
If you're looking for a car to build and enjoy, then it's a matter of placing the monetary value on what it means to you.
If you're looking for a car to build and flip then it's a matter of placing the monetary value on what it means to someone else. With the deals out there now, I can't see you getting even half your money back unless you play speculator and sit on it for a while to see where the market goes.