Try to go for a tall one where your cat can comfortably reach / stretch up and scratch on it. A bed on top of it would make a great viewing point for a cat that likes to be elevated or as high as possible.
You can place this in front of a scratch prone couch to try encourage the cat to use the post instead of the couch.
As for horizontal scratching, those cardboard scratching boxes are great for this. You can chuck em anywhere in their walkway and they'll use it as they go about their day. Makes for a good alternative bed if the cat is feeling like it.
Letting dry on the balcony will do, especially if its in the sun. The aim is to disinfect, not just clean.
I just hose mine off, use household disinfectant as per instructions and rinse off. If I'm in a rush I'll dry with a rag but leave it out in the sun if i can.
Regarding litterboxes, I have heard metal is easier to keep clean as the plastic ones will get scratches, which will harbour the bacteria. Personally haven't had any issues yet as long as I clean it properly, and it's also not easy to find in stores except online. So what you get will do just fine.