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Adding my 2 pesos to your inquiry I'll pretty much ditto what TexMex has added. Darn good points to keep in mind when you make your decision.

My recommend is you look for a good, used .38 special revolver with a 2.5" to 4" barrel. There are many of these around that can be picked up at a fair and good price.
If you're dealing with a reputable gunshop, tell them you'd like to see several. If they don;t have any in stock, ask them to keep an eye-out and call you when they get a "good" one in. Ask friends, a lot of times these can be bought from widows and surviving spouses - "Just something that I found in a drawer" type thing. (sadly these days are almost gone)
At the gun shop ask about the condition of the "cyclinder lock", the "forcing cone" and the "timing."
Maybe you don't know what these are all about but it'll give the guy the opinion that you you do and may save you some trouble in the future...;).

AMMO - target loads for the .38 special are easy to shoot, low cost and fun. You and your wife should shoot at least 2 boxes each just to 'get a feel' for how the pistol shoots.
For home defense I recommend 125 grain Black Nyclad JHP (jacketed hollow points) aka the old "FBI load." For defensive purposes, simply said, it works.
When shooting, don't try to get to fancy - Aim for the middle, fire 3 rounds and see what the result is. Repeat as necessary.

As always, practice, practice practice. Its fun. Its a "bonding thing" with the wife. And it could save your lives if worse comes to worse. Think muscle memory.

Don't worry about all the bells & whistles fancy schmancy semi-auto zircon encrusted rastafarian bling bling speed-loadin' euro-trash top-of-the-line whizz guns. Start basic, learn the weapon and learn what to do. Its all about something going 'BANG!' when you squeeze the trigger.

And yes....45 is better. ACP or LC.