The expansion tank does not go ON the hot water tank, it can go anywhere in the system after the prv that is convenient. Since the pressure basically is the same anywhere on the house side of the prv, find a place where you can place it where you have easy access to check it. I've only had a limited experience with the things, but my first one lasted about 12 years. Took me all of about 10 minutes to unscrew the old one and install the new one and (if I remember, maybe about $20). Because of the diameter of the things, I didn't need any tools to unscrew or install the new one - you have enough leverage to do it by hand (at least I did). The best way to do this is to install a brass tee in the water line, a shutoff, then the tank. The shutoff makes it really quick and easy to replace when the time comes.