Hi Irish, Lots of questions and you're in the right place. I would say search the forum for terms around what you're trying to decipher. In other words search on SS material or plate size.
Much of what you're asking can certainly be found here and there is always more than one way to skin a cat.
If you have some burning questions, ask those individually instead of a smattering like you've done. Too many issues here to address properly. Sound like you simply need to start reading

and then ask questions as you go. -Small digestible chunks.
I'll start with one question: 316SS is the most corrosive resistant SS within our budgets we're usually working under. It's available and common so we build with it because it will last the longest of the common SS we can get our hands on. Other SS will work but it degrades faster.