Rebuilding a power steering pump, like a water pump only makes sense it you really want to do it for the experience. In practical experience, it is better to buy a rebuilt unit due to how easy it is to mess up the overhaul the first time and have to do it over.

Often, these kinds of parts are designed so it takes special tools or fixtures to assemble them properly. Another consideration is that you are messing with a critical safety part.

If quality and durability are your goals, buy a new pump instead of a rebuilt one.