An M37 is very collectible, and fun to drive. Engine parts are easy to find, but body parts rust away and are gone forever. The gauges and wiring are fairly common to that era, but the old style Douglas wire connectors are getting hard to track down at a reasonable price. Any M37 with a winch is often missing the winch since it could be moved to other trucks. Many of the common parts are being reproduced, though, but at higher prices than we used to pay for NOS parts. Canvas is being reproduced, but again at a higher price.
You can rebuild anything, but at a certain point your wallet will start to scream. A good general rule is to track down the best unrestored truck you can find at a reasonable price - don't buy somehting that had been modified all over the place at a premium price for the mods, then decide to restore it back to original. At that point, you are throwing away the money you paid for the unwanted modifications.
Years ago, I looked at an M37 that had become a rich man's toy - complete with yellow Imron paint, a larger motor, an auto tranny, 12 volt conversion, and all for only $ 11,000. I walked away and found one parked in a construction company storage yard for $ 750. The price was low since the engine was worn out, but I knew that from the start. The engine rebuild was a known factor that wasn't a surprise. I planned on rebuilding the differentials, the tranny, and the transfer from day-1. When I finished, the entire drive train had been rebuilt, and I had a reliable truck. If you buy someone else's restoration, you are relying on them to have done things the right way, and you might find out later that they weren't much of a mechanic.....