I also have a 503 and 2 - 400s. I really like both of them. They are basically the same except for looks. The weak spot on the 500 series machines is the top and the "face" cover. They are weak, often broken and cannot be replaced, so if it's broken, know that you can't fix it.
I would offer her less, but if I didn't already have too many machines, I'd buy a nice one in a good cabinet for $100.00.
bkay