ObsidianShadow

I am an aircraft mechanic and most of my service manuals and ADs are in pdf files now. Printed copies are hard and expensive to come by. So, I'd like a cheap tablet solely for the purpose of having these pdfs on hand when I'm in the hangar next to the airplane. Here are some ideas of what I'm looking for:

I'll use it only at work, so I am willing to compromise on these:
-Cheap! ($300 or below preferred, $500 or below is do-able)
-It doesn't have to be new, a used or refurbished tablet will be fine
-Rugged (or good cases and screen protectors available for the model)
-Stable, no horrible bugs or known frequent crashes or lock-ups
-7 or 10 inch screen preferred
-Backlit/Color screen
-Not too heavy
-WiFi is preferred, but not necessary if the price is right.

It doesn't have to run apps or games like lightning, just reading pdfs and annotating them (ezPDF Reader app) is all I need. I need it to be rugged or at least have a good case and screen protector because it will be stored in my toolbox and used in a dirty, greasy shop environment.

Any ideas? Thanks guys!