The nut has a special ramped slot, which accepts the hand crank, and then pushes it away when the engine starts. If it doesn't start, repeat until it does. The crank will automatically eject from the nut when the motor starts. It really doesn't matter. Get one that appeals to you.
Earlier busses are cool, but parts can be more expensive and harder to find.
Not to be a prick, but I can't possibly know where all of the busses for sale are. The very nature of this sort of information is transitory at best, and is highly dependent on where in the world you are. I can suggest that you dig up every newspaper in your area, and try to get the newspapers from the larger metropolitan areas around you. As a rule, the bigger the city, the better your chances of finding a bus. The best thing to do is to attend a couple VW events, so you can see other busses and get a feel for what you are getting yourself into.