First, welcome to the Board. I am originally from South Jersey, born and bred and grew up there. Others have given you very good advice about buying used from a local dealer. I have a QE 153 Bernina that I bought used from my local dealer and have been very pleased with it. Have had it for years. I especially love the knee lift on it. However, I also have a new-to-me but older, used Pfaff. That has the built in "walking foot" called an IDT but not the knee lift. My dream machine would be one that has both those items on it. I know the newer more expensive Berninas now have the IDT because the patent on the Pfaff IDT has aged out and I think the newer, more expensive Pfaffs have the knee lift now. When I win the lottery I will be able to buy one of those but too expensive for me right now. So, it depends on how much you have to spend. Maybe go to several shops and see what they have used for sale and what extras they come with. The Authorized Sew & Vac store in Collingswood carries and services a number of different machines including Bernina and Pfaff. You can google them and see the list. They do carry other good name brand machines that others like a lot. There is also a shop in Pitman called Mister Sew-Vac who also lists Pfaffs but I am not sure about Berninas. Google them also.
Good luck with your hunt. Hope you can get some hints from your Mom as to what she would like to have. That would surely help thin the search. If you also sew, try some out in the shops. If not, have them demonstrated for you. I would say you could check Craig's List in your area but that is a hit or miss opportunity especially if you don't know exactly what you are looking for or know how a specific machine should work. Best to stick with the shops.