I believe this is a good machine though I've never used one - a relative sells Pfaff machines. I have the QBot on a Tin Lizzie, and though it's nowhere near the top of the robot range I love it as the designs are sewn out so accurately. I have so much trouble manually with circles and arcs, they always end up with corners.
Get the owner to give you a good demo of the machine working, try it yourself, and ensure you have manuals. Also keep in mind you will want software to create your own designs and/or put them on the card for the QBot. I use 4D Quilt Design Creator from Pfaff. You can teach yourself, I did.
As others have said, think of the access to a good technician. My dealer is an hours drive away, and they won't come to me - neither will the local techs.