Welcome to the board
First you need an engine, then add the load of gear and people, and using a tachometer you have to purchase and install, take it for a run and see what max RPM is. This will help determine which prop. If buying a new engine and they are offering an upfront prop switch, I'd recommend asking the seller for advice, as they may be highly knowledgeable.
Engine trim, which is the angle of the outboard, is just as important. Usually around the second hole works to start.