Since FMQ is your primary goal with this machine, I agree completely with taking a test sandwich. As well as at least a small quilt to work with. If the sample sandwich test goes ok, move on to the quilt test - even if it's just a throw away quilt for this purpose only. Some dog will love it!
If you are familiar with this particular dealer and/or tech, I would trust the tech's word as to reliability. Most used machines do not come with a warranty. Hence my earlier suggestion to negotiate on price. Personally I'd probably start about $350 and work my way up. They have all kinds of wiggle room on used machines. Especially if you are willing to walk away from it. I'm a cheap witch. I start very low and have my top price set in my mind - as long as I'm willing to walk away. If not - then I'm willing to pay whatever the final number ends up being.
Just my opinions.