Matt the tightness of the clutch shoes depends on the springs. The stiffer the spring the higher the engine rpms have to be before the clutch engages. You can adjust that be using stiffer or softer springs depending on your driving style and the surface you are running on. The springs do wear out and break after a while so the one thats on there could be softer or just worn as you said.
The CB's are pretty much standard. The only ones Ive come across that are different was a traxxas one. If you stay with HPI CB's you'll be fine.
What washer are you referring to on the collet?
When you seal the engine, dont forget to do the back plate as well as the carb.

tcoop, the old flywheel should work fine. If the teeth on the spur are worn definitely replace it. Stick with a plastic spur since they are cheap and you want a weak spot in the tranny so that you dont break something more expensive if something gets stuck.
I would definitely upgrade the diffs to the hardened ones. They are more expensive but in the long run it will be better then having to keep replacing the cheap ones. One good 180 on throttle and you are going to shread the stock diffs.