I agree with everyone else a.) that you are a total sweety for doing this, and b.) that Mom should be part of the decision. Although I have Pfaff machines, I would recommend that you look at Janomes.
It is very nice to have even-feed fabric travel, needle up/down position, needle and stitch control and a few other bells and whistles. Particularly if your mother has been working with an old machine, she should have the opportunity to test drive the machine to try it out.
You may be interested to know that sewing machine shopping at a dealer has room for negotiation. Don't know if you like that sort of thing, but it is possible to negotiate the price.
Why don't you narrow the search, make some print-outs and wrap those up. That way Mom can spend some time to research and you could go shopping together.