You login to MUSE and can just pay on a debit/credit card. I think you start off with Β£2 by default.
There are student printers everywhere (ish). The main pitfall/things to know is that they try and encourage you to print double sided and that colour costs a lot more.
There are some printers that are staff only, but you're actually quite unlikely to come across these. If it isn't a big bulky printer which looks like a student printer then it isn't. The staff ones tend to be a bit smaller and seconded away in random places.