Caliber wise I would recommend 9mm, for the following reasons
500 to 750 will get you a lot. But if your are looking for a good reliable gun for a good price you can get the Bersa Hi Capacity 9mm Pro.
at around 400 it is a GRATE pistol, just look at my avatar. Accurate, Light weight (aluminum frame) and in your price range you will have lots left over for ammo.
As for the XDs. They are top line pistols, XDm is the next step up if you are interested in competition, but many people use the XD for that as well.
Glock and Walther P99s are good guns to look at. All are well priced and totally reliable. I have them all and have never had issues with the guns.
If you wish to stay with the Springfield line and you are looking for home defense and something you wife can handle, Try the XD Service model. It has a 4 inch barrel so you could probably CCW with it in the future. You could also buy the Service model ported and buy an extra unported barrel. You could use the ported barrel at the range to help your wife learn to control the recoil, then when she is ready you can put in the unported barrel and go from there.