Wow... that sounds interesting...
Originally a 24' extended to 32'...
My initial reaction is that it's a one-off custom coach, and you'll not have a clue about whether it's a lemon or a keeper until AFTER you buy it. It had to have 8' of frame installed in the P30 chassis, so you have at least four welds that may or may not have been done properly. You'll also have some drivetrain components extended or added. Skin had to be spliced in.
Obviously, it's not falling apart at this point, but who knows... I certainly don't think it's worth more because it's a one-off.
My suggestion would be to have the frame and drivetrain closely examined by a truck mechanic for soundness. If it all checks out, offer the owner what YOU think it's worth. If he takes it, you get a deal. If he doesn't, you're not stuck with the same white elephant he's stuck with.
There are enough factory-original Argosy and Airstream motorhomes for sale out there in good original condition, or restored condition that you don't need to jump in with both feet if you're not prepared to yet.
AIR 2053 Current: 2006 Born Free 32 RQ Kodiak Chassis, & 1995 Coachmen B-van
Former Airstreams: 1953 Flying Cloud, 1957 Overlander, 1961 Bambi, 1970 Safari Special, 1978 Argosy Minuet, 1985 325 Moho, 1994 Limited 34' Two-door, 1994 B190 "B-Van"