The thing is i may buy new,if i do the D610 gives best value for my needs,if i go secondhand that opens up all sorts
The 610 may be all you may need, and the layout and feel will be very familiar to your current camera. So your transition would be instantaneous.
Technology only yields small advantages between the two. Maybe a more efficient sensor, ability to print larger images, better ISO for dark situations. However me myself do not find new technology to have that greater advantage over tech from just a few years past.
The same way a Countach would still be a welcomed addition in any garage even after 20+ years, and greeted the same way an Aventor would.