That's a deal. The 3" will be fine and it should have 5 chokes included, your BIL can help you choose the best choke for your outing or you can pattern it if you have time.
Is it the Field (ComforTech) or the American M2? If it's the American They an be bought new for that price.
American
http://www.benelliusa.com/shotguns/benelli_american_series.php
Field
http://www.benelliusa.com/shotguns/benelli_m2_field.php
There are 2 nipples screwed into the stock that the pad clips on to. It's kind of hard to get off. Don't try to pull the whole pad off a once, start pulling either the top or bottom do 1 nipple at a time.