^^^^
Agree, especially replace all O rings. I do this whenever I get a new used keg, and you'll want to do so once in a while in the future.
On general keg cleaning: I break them down every time I clean them. I've learned from long experience it's way easier to give a wrench a couple of twists than to come up with some elaborate CIP method. Fill the keg with cleaning solution, drop the liquid dip tube in there, all the little pieces and the lid into a little pail of cleaner. Come back and rinse and reassemble whenever you get around to it.