Most are valued between 700-950 dollars in the condition that you describe.
1977
Impossible to answer without a DETAILED description of ALL markings, barrel length, choke tubes, sights, condition, box, accessories and papers.
Remove the forend, then the barrel. The serial number will be on the underneith side of the bottom barrel at the point where it hinges with the rear stock. You may need a magnifying glass.
You need to seek the services of a qualified appraiser.
Ruger red Label over and under 20 guage in very good condition, what is it worth 300-475 USD
if box is good condition, and all shells in it...100$
Yes. Ruger has sn data on their website.
Main consideration is fitment of the gun to her size. She must be able to hold it correctly and comfortably to handle the recoil. Without proper fitment a 20 gauge can feel like it kicks worse than a 12 guage. My wife shoots a 12 gauge Ruger Red Label with a 26" barrel and loves it, perfect gun for her small frame.
600 or so
marketing ploy
100-1000 depending on specifics