Couple thoughts, I have dual Power Pole Pro's series II installed on my boat. First realize that pre-series II poles are way behind on soft ware and hardware. This means if you want to add another pole in the future, they can't be linked, I believe one remote can be used for both but syncing and diagnostics may be an issue. I got mine in 2015 and I'm already behind on the operating system. I have 1.0 and the new is 2.0 and they are not compatible with each other.
Power pole does have the best product support team, bar none. But you may want to look into part availability for pre-series II poles. I love my poles, don't get me wrong, but I have had issues with them. Power Pole has always backed their product and sent parts right away at no cost, but again mine are under warranty.
Dual poles are the way to go in my opinion, so the ability to add a second one later would be paramount. This is where the compatibility factor comes into play.
$600 maybe a good deal if it is in working order, the mount alone would cost about half that. Look at the hydraulic cylinders for leakage, hardware for corrosion, and condition of the pump. I would see if it could be connected to a battery to ensure its operational.
This is just my opinion. If one pole will work for you and you don't plan on upgrading to two poles in the future, this may be a good deal. Again, check and see if parts are available. I'm sure they are.