If it is a lower hour machine $650 isn't that bad.Cheaper than buying a newer machine with a light duty transaxle and one of the newer Kohler courage engines in it for $1600-$2000+.
Since there isn't an hour meter on them you till have to go over it and look for wear.Inspect the deck,deck lift and steering components and look for wear.It will give you an idea if it's well used or lightly used,well cared for or used and abused.
Looked them up and they were $2200-$2600 new.
My cub cadet HDS 2165 with 46" deck showed 1662 hours on it but I'm sure that the key was left in the ON position a several times.It more than likely had between 600-800 hours on it.Steering was a little loose in the tie rod ends and the valve covers both needed to be replaced but other than that it was a strong runner.Mower deck was in great condition.I payed $450 for it and sold it for $600 a couple years later with a New PTO switch and relay,new deck belt,battery,valve cover gaskets,fresh fluids,filters,4 new tie rod ends and a good bath and wax.
Person that bought it is still very happy with it.He's my neighbor and uses it weekly.I told him when he bought it to keep it clean and keep up with maintenance on it and it will last him many many years.