Among the several good suggestions that you have received, the Beretta 92/M9, Beretta PX4, and CZ ideas stand out in my mind. 9 mm is cheap ammo, easy to learn to shoot well, and still performs well as a defensive round. Your wife might be pleased with a thumb safety. The CZ can be hard to find in some areas, but is very well thought of. The 92 is usually not hard to find, but may be a bit over your budget; very fine pistol, though. The PX4 can be had new for less than $500 if you shop a bit, and its rotating barrel makes it extremely soft to shoot. My daughter has the PX4 Compact in 9 mm, and it is a little gem.
As others have said, though, your decision can ultimately be made only by getting your short-list pistols in your hands, and preferably by shooting them.