To put training in perspective, I was an Army officer and foreign weapons instructor for my unit. and an NRA Pistol Instructor. I still took a three day CCW course and take a refresher every couple years. It's not something to skimp on and you need to know the laws governing the use of lethal force.
Personally while revolvers have some advantages, a semiautomatic pistol is flatter and the manual of fire is no more complex. There are some very small, lightweight pistols that may fit her hand.
Smith & Wesson has Shield and Bodyguard models, as well as the M&P (Military and Police) Compact.
Ruger makes the LCP series.
A few tips. Many of these pistols are designed for women and/or have changeable grips: it must fit her well to be accurate. Most assaults (something like 80%, not sure) occur in low light conditions so high visibility sights make the weapon much more practical. There are specially designed purses and pocket carry holsters but she needs to be comfortable acquiring the weapon as when under threat it may be harder. Finally, there are many conceivable situations where an incapacitating spray is a better self defense choice than a gun, so having both is a good idea. Also you can usually take those sprays into places that prohibit firearms.
Buying used can work but most people go to a gun shop. Some gun shop staff (like the owner, usually) are knowledgable while many gun shop employees are poorly trained or completely untrained, so don't hesitate to walk out if things If you know a state trooper or sheriff or local police officer they will often give you and your wife good advice, both in terms of equipment and laws.
Training courses aren't too expensive if you are buying the ammo, and some will provide a weapon at no charge. We're fortunate to have the Sigarms Academy in SNH, and they let you try any of their pistols in any course, and give you a discount coupon to boot. You may have a similar facility nearby.