The older, electrics, like Singer 15-91 are good for denim. It has a vertical feed bobbin that can be used for free motion quilting. Denim is a once in a while thing and she will probably move on to softer, 100% cotton fabrics, which are the main quilting medium. The Janome Horizon, used by Leha Day in her videos is a great quilter because of the DUAL FEED system and the automatic thread cutter and needle threader. Dual feed keeps the fabric from creeping under the needle and it delivers the fabric at exactly the right speed to the needle. They list for over $2,000, but you can get them for a lot less now that a newer, more fancy machine has come on the scene. Personally, I would get a Janome of some kind, the best you can afford. The parts are more reasonable than the European machines. Attachments are readily obtainable. Whatever you do, please, get her a 1/4" quilting foot with the metal guide. It makes 1/4" seams more attainable without eye, back and neck strain.
Quiltmaker magazine, 30 year Halloween edition, has a chart comparing all the quilting machines. They are more toward the high end pricing, but you can see the features. Rather than suprise your wife, sit down with her and go over the features of various machines, so she can get exactly what she wants and you will be rewarded by her contentment with the rather large purchase. Good luck and thank you for being so thoughtful.