Some bits of advice:
1. Buy the nicest one you can afford. The engine and drivetrain on these is not too difficult to find parts for but the body and chassis is unique. I wouldn't even get out of bed to look at one of the $5k ones as it will cost a LOT of money to bring back. Think in the 10-15K range for a GS and well north of there for a radio body as they are rare this side of the pond.Ambis generally go cheaper and make great expedition vehicles.
I haven't ben able to locate either a radio, or an ambulance to even see a picture of it.
2. Check the front axle as the chrome knuckles are integral with the axle tube and replacement is a 4 figure number.
Do they all have the chrome knuckles?
3. Join the club. These trucks have an excellent club which has a very active parts remanufacture program. They have stainless fuel tanks to replace the ali one that WILL rot out, new door tops, fluid reservoirs etc. They are looking at new chassis and wiring harnesses among other things.
Which club? Do you have a link?
4. Find one and ask for a test drive. These are military trucks and make no concessions to comfort. Generally you either love em or hate em and most owners are fanatics for the type.
Where have you been looking and how far will you travel to get one? I may have a friend of a friend looking to sell.