I need to replace a 15 year old Rheem Power Vent water heater that is leaking. I plan on replacing it with a new Rheem water heater. I think I can do this myself. In looking at my old unit, it appears that the power vent is just an add-on motor and vent on top of the water heater. My old vent works fine. The new Rheem Power Vent water heater costs about $400 more than the non-power vent model. Can I simply buy the non-power vent water heater and add my old power vent to it and save that $400?

I doubt it. The power vent is actually a specialty double gas vent: it pushes burnt gases out though the middle, smaller vent, and sucks in oxygen through the outer, larger vent. With every year, there are changes in design. There have been lots of changes in the last five years or so and all heater I know have some changes done to them. For your own safety, get a new vent dedicated to the heater you'll buy.