I was looking at some of the OS 4 stroke engines on tower and noticed that the pumped engines are a lot more expensive. Do these pumps just have a diaphragm or is it more complicated than that. What is the general rule for determining if you need one? Is it solely tank position/distance from the carb?

Tank level vs. carb level and greater distance apart than normal would be two good reasons for using a pumped engine, but typically a good pumped engine will run with a constant fuel mixture throughout the entire flight envelope and regardless of the fuel level in the tank.