Pilgrim makes good points about accessories and barrels. You can also consider that the 870 can be had in a left hand model.
http://www.impactguns.com/remington-...700255774.aspx I don't know if length of pull will be a problem, since you describe your wife as a tallish slender woman. If there is a problem and one of you need a longer gun, but you are to share it, the quick fix is a slip on but pad.