OMG! If he will give you a 6 month warranty or such, $600 is a great price, and lower is greater!! You will love the machine. It is a workhorse!! Grab it!!!
The big difference with the 6500 vs the 6600 is the built in walking foot and the larger harp (opening) on the 6600. For that kind of price, you are well off to buy the 6500 and just use the separate walking foot.
All Janome's are great. I am a great fan - the value is there!!