I've run into quite a few varieties of swords, helmets, clothing and shields.
Is there any way to know which is better? Or can I just assume that they get
better as the levels get lower?
As a general rule (stress _general_), that is a safe assumption; However, keep
in mind that there is enough stuff down there for 4 characters to be pretty
evenly equipped (except for the 2 suits of plate armor), so lower does not
_always_ mean better. A better guide is usually common sense (chain mail is
stronger then leather, plate is better than chain, etc.).