almost all guns have the gauge marked on the gun somewhere. If not, you should probably take it to a gun dealer and let them look at it for you.
Good shotgun
$50 more or less.
100 USD
No
You have stated what you have, not asked a question.
http://travel.webshots.com
100-300 USD
Try e-gunparts.com
Follow the instructions from the manufacturer.
Post the serial number of the gun and I'll let you know.
Blue Book of Gun Values will help.
50-200 usd