I have one of the these sitting on the shelf as well. As the car is MOW I'd think you can paint it any way you care to, always remembering the roof covering was canvas and likely heavily faded and weathered. Some degree of peeling paint over varnish for the sides and liberal dust and grime for the body. Personally I wouldn't over do it, I prefer the "used but not abused" look for MOW.
As to prototype, the Ambroid kits date from an era when "freelanced" was more acceptable than now. Plausable is close enough in this case IMO.
Have fun and post a pic when you are satisfied.
Randy