You will not be disappointed with the 6500; it's a fantastic machine. I will say that many dealers are taking 6600's as trade-ins for 7700's, so you may be able to get a 6600 for the same price or less than the 6500. I wouldn't have any hesitation about buying used as long as it's from a reputable Janome dealer who offers a warranty on the machine. I did buy my 6600 through ebay and never had a bit of trouble with it but you do take a chance that way. For me, the savings (I paid $900) was worth it. In fact I still have it as I traded in my 6500 to get a 7700.
All three are great machines!