May I ask a few questions?
First, what sort of riding does your wife want to do?
Does she have any health issues, like back problems?
Has she ridden before?
Where do you live (rural or in an area where traffic can be busy at times).
Do you have room to keep your horse at your residence or will it be boarded?
If your wife wants to trail ride, then many breeds will fit the bill. You don't want a very young horse, nor one that is senior; a teenager, if you will (8 yrs to 12 yrs).
If your wife wants to participate in events such as field trails, then you want a gaited breed, such as a Tennessee Walker or Missouri Fox Trotter. It is difficult for a non-gaited breed to maintain the gait of a Walker or Fox Trotter and you will find in events like Field Trials, gaited breeds are ridden. (http://www.gaitedhorses.net/Articles/FT ... rial.shtml
If your wife has any back problems, the gaited breeds again are an excellent choice, offering a smoother ride than your non-gaited breeds. You would have to ride one from each class to understand this.
If your wife may be riding where cars, trucks, motorcycles could pass by, you want to be sure the horse does not spook easily.
I strongly suggest training, not just for the horse, but for your wife as well. If she hasn't ridden before, or it has been years since she rode, training is in order and the best for horse and rider alike.
As a personal preference, I prefer geldings over mares (you do not want a stallion). Mares can be termpermental when they are in their heat cycle. A gelding is the same, every day.
If you don't have knowledge on horses and the "lingo" when buying, riding, etc, take someone with you that does. I am sorry to tell you that I have not met a person yet that was selling a horse, that wasn't a bit dishonest when describing the horse. Trust me, if the horse was a wonderful as they are going to tell you it is, they wouldn't be selling him.
If you have any questions you'd like to ask, feel free. I'll try to answer as best I can or direct you to someone that can.
Good luck. Horse ownership can be rewarding!