OMG! Sounds like vintage machine paradise!!!
Vintage Bernina's are absolutely wonderful machines and have all the glamour, as far as I am concerned. Vintage Pfaff, Singers and Viking can be outstanding. You may want to limit yourself to looking at a particular brand (Singer is popular) that has the feed dog and other characteristics you want (post here so people can give you the model numbers to look for).
I would probably bring along a very long extension cord (so you can plug a machine in and make sure it works without having to lug the machine over to an electrical outlet), some machine needles (standard Singer from JoAnn's and also the more modern machine needles), thread, threaded bobbins (I would purchase a few bobbins for class 15 machines and also class 66 machines; both very common for Singer and other manufacturers), scraps of quilting fabric, small scissors, and a cheap needle threader.
In terms of narrowing down your search and keeping costs down, I would start by eliminating any machine that is not complete (you need both the electrical plug-in and the foot controller). I would also skip over any machine whose wiring is brittle, damaged, or really really old. Obvious rust would also probably send me to the next machine.
Next test would be to look for the bobbin case (unless it's a top-loading bobbin). Bobbin cases can run $25 to $35 with shipping to replace; sometimes more for authentic vintage cases that tend to be better than reproduction cases. The bobbin case is what you put the bobbin into.
The next thing I would do is turn the hand wheel to make sure that the needle goes up and down. If you can't turn the wheel by hand, go to the next machine.
Next test would be to plug the machine in and make sure it turns on. At this point I usually remove the needle and then run the foot controller to make sure the foot controller works.
Last step is to actually thread the machine and sew on fabric, to make sure the machine forms a stitch. If everything else is okay but the machine does not form a stitch, this usually means a trip to a sewing machine technician to get the timing reset on the machine.
With that many machines to check out, you could determine before your visit exactly what model of Singer you want to look for. That would help reduce the time you spend looking at machines. Me, I'd be looking at any Bernina's first!