I always replace the pressure relief valves on all my used kegs. I usually don't replace poppits unless needed(too expensive). Also get yourself one of the long skinny brushes meant to clean the inside of the liquid tube. You can find one for just a few bucks. My process is that I disassemble the keg 100%, throw all the parts inside(including the lid. Top off with hot water and pbw and let it sit for a few hours. Scrub it down with a brand new toilet brush, and use a toothbrush to clean the posts, and the long brush I just mentioned for the dip tubes. Drain the water, rinse and dry, then reassemble with new o-rings and PRV. Haven't had a single infection, off-flavor, or leak to this date.