What a lovely gift. What is she sewing on now? Has she been checking out some specific models?
This is a personal choice, of course, but I will throw out an initial suggestion. Try comparing a few on a site like SewVacDirect to learn what features are available. For example, the Brother SE1900 looks like a lovely machine with good features and fits your budget. Brother is known for their friendly embroidery systems. It has a nice color screen, a clean look, thread snips, etc. Looks like a great all-around machine.