If there is any such thing as an average price for a 72 that has sat a while and looks ok but otherwise in unknown condition I would say $2500 +-.
A month or so ago I bought a 75 Sovereign
with a similar story for $2000.
Don't let your heart get ahead of your heart. Expect to put in a lot of time and money. The selling price is just a door opener.
If the Overlander has been sitting near the Gulf there could be a lot of structural corrosion you can't see.