Blue Book of Gun Values will help.
100-300 USD
Good shotgun
Take it to a gun smith.
$50 more or less.
No
100 USD
You have stated what you have, not asked a question.
Follow the instructions from the manufacturer.
Try e-gunparts.com
http://travel.webshots.com
Post the serial number of the gun and I'll let you know.
50-100 or so