Buying a sewing machine is much like buying a car. You need to take it for a test drive to see if you and the machine are compatible.
You could call some dealers in your area to see HOW many machines they have in your price range. If they have quite a few then tell them what you want to do and they might have ideas to help you with a surprise for your mother.
OR-you can get a pre-paid charge card, put the money on that card and give that to your mom, THEN the day after Christmas, you and your mom visit several dealers and let her choose the machine she would like. A gift certificate limits her to the one store. The pre paid card lets her choose from different brands. And if YOU take her, then you know that the card money will go on a machine AND it gives you and Mom some quality time together. Trust me, MOMS LOVE THAT!