For me, buying the entire pump assembly paid off. Considering my previous unit had aged and needed servicing throughout it was a better purchase to replace the assembly.
It is really easy to replace and with the bottom seal included was much faster than a rebuild. Prior to replacing, I did flush and sanitize with bleach as best I could so that the mess and smell was kept to a minimum. Do at least wear rubber gloves and be ready to take a shower in case you splash yourself. Take your time and you should not have any problems with the replacement.
From a price perspective, it was cheaper to buy the whole unit rather than all the replacement seals. The unit came with the bottom seal and a new joker valve. Other than perhaps needing new clamps everything need came in the box. As suggested, I did leave the original bottom hose port on the original hose. I removed the hose port from the unit and pulled out the joker valve and placed it in the old hose port and then screwed the new unit to the old hose port. Then screwed the new assembly down to the base, connected the top hoses and I was done.
This is a simple task that just about anyone can do. Just as a reminder, do NOT tighten the screws to tight as you may break the plastic and remember, someday someone will be doing this again.
For me, I recommend getting the entire assembly every so often.