Agree with the others, if you want it, you should go to her place and know that it is working .... plus that when you get it that you have everything! Too easy when she is packing it up that something gets left behind, such as the bobbin winder!
The number of stitches is negligible, so no need to sweat on that, so long as she is telling you the correct number.
Price, agree too, without a warranty behind it (that you could maybe get with a dealer) does seem high. All I can say there, is to double check the current prices for the full package, and make your decisions on that.
Another thing, check which S16 model it is. Aren't they onto a 3rd (or 4th?) generation by now? Almost the same machine, but some nice features that you may wish you had, and would sooner pay the full price for an up-to-date model.
While you said you really want the machine, you didn't say whether you have done any test drives and how familiar you are with the machine. Regardless, I would spend some time with a dealer, and get really familiar with the machine before I headed off on this possible venture .... or decide whether you want to buy all new with warranty, training, support and service. You're missing all of those in the re-sale.