I have two Aria Pro II currently.
There is a ton of Vintage Aria, Aria Pro II on ebay.
Better quality back then than the cheap low end guitars you get today. Even better if it says Matsumoku on it.
Just make sure its got an original body I've not seen a clear finish Aria was known for using stains. This guitar by now should be showing pitting on its metal parts.
You would probably find it hard to come by some good examples of the exact model he has. If it is from 1977 I would be asking where he got that info from as it looks newer to me.
It won't be a lawsuit guitar unless there was an actual lawsuit.
If its great condition for the age then $400 is reasonable but remember you could pick up a new MIM strat for that a few months ago.