I always preach comfort over everything else. I don't think there is a minimum calibre for self defense. I've seen a .22 do plenty of damage.

If you're fine with 9mm, then I say go for it. In the heat of the moment, you want accuracy and dependability above all else. When you throw in that cost is an issue, a 9mm makes even more sense for you. I'd suggest you not carry anything of a higher calibre until you've become as comfortable with it as a 9mm. A .45 going off in an untrained hand is quite a lot to ask, especially for your wife.

That said, consider self defense rounds in the gun when it's at home or when you're carrying. This will add to the stopping power. I am a big fan of Hornady TAP self defense rounds myself but I know a lot of folks who prefer Federal Hydra Shock.

I carry a Glock 19 Gen4 most of the time (and sometimes carry a Kimber 1911 CDP when Im feeling like a .45) and it's a great weapon. It will always go bang and if it ever breaks, Glock will back it up no questions asked. I highly recommend you get the Gen 4 if you can find it. Lots of upgrades, smoother trigger pull and a redesigned spring.