I would let the tech go over the machine and do his/her tune up and then re-assure you that everything is perfect before you agree, although maybe put a hold on it pending that. Also, will there be a warranty?
I have a Janome 6500 and it is a workhorse. I wouldn't hesitate to suggest the brand to someone.
As to experiences buying used, I bought my mid arm used off of Craigslist from a lady who was down-sizing her home and it has been perfect in every way and was less than 1/2 the price of new, so deals are out there.
Be sure to negotiate the price or accessories with the dealer.
Watson