> but I'd like
>to know if this is something I should always be on guard for
>or if it's a rare occurance that will become even more rare
>when I start to look like I can defend myself.

It happened quite often to me with my first evil characters, especially when I had some valuable gear.
Once you look competent enough it will stop happening.

>Finally, are there level limits on items you can wear? It
>seems like when I find stuff lying around on the ground, I'm
>able to wear it all.

No limits on stuff you can wear but you might not be able to use the equipment to its full extend.
Exception:
-Wands,Scrolls and Talismans will not work till you have the according level.
-You can only buy gear that is ten levels or less above you.