You have a good go-by. Check out the
thread from the eBay/Criagslist forum where there was a 1950 GMC ice cream truck that just ended auction (reserve not met.) Try to make contact with seller and learn what problems he had. See if you can find out where the bidding stopped to find out what the approximate value is. This can be a starting point to find out how much you want to put into it.
The DQ ice cream truck is quite a project. I believe that you'll turn heads if it should complete, but remember that, when you're done, you're going to have an ice cream truck. This is a very specialized ride.
Keeping it stock, you can follow advice in the "How To Restore Your Chevrolet Truck" book. If you go with a modern drive train, you are making a one-off. In my opinion, you're also undoing much of the historical value of the truck. (A scan of the Mona Lisa isn't as valuable as the painting.)
I'm simplifying things, but in the end, it comes down to the depth of your pockets and interest. Hopefully one won't run out before the other.
This restoration will be everything that all Advance Design trucks would require PLUS the ice cream-iness.
Keep us abreast of your efforts!
Bill