Minis can be great for people who love horses but for one reason or another, can't handle a full-size one. Rather than look for a baby, who might give your mom more problems, look for an older mini with good manners who just needs someone to pet and love on them. She might even be persuaded to drive one, given enough time.
Minis are like any other horse. When people don't discipline them or train them, they can be the chihuahuas of the horse world - nasty, undisciplined and ill-mannered, just like a yappy little purse-dog.
I would look for a mini from a reputable breeder who handles their stock daily, or a mini that is already good on ground manners and possibly drives- many breeders will help you find stock that is pet-quality, animals that have great handling and breeding, but who just aren't cut out for the show pen for whatever reason. If your mom is timid, she needs something that is going to be enjoyable to work around from the get-go.
I hate it when people buy minis going "Oh I'm going to train it to _______". They will turn into the project that never quite got off the ground when you've got bigger things to worry about. Buy one that is already trained for what you want to do, especially for a beginner.