It's going to be (my guess, and we KNOW what that's worth...don't run to the bank with it!!! ;) ) either a 27, 127, 28 or 128...hard to tell from the photo they show of the machine, and cutting off the faceplate in the photo.
Pretty machine...I'd make them an offer...ask about attachments, extra bobbins or shuttles, motor, etc.
It doesn't necessarily need a cabinet to work, it depends on whether or not it's in a case...