What should I be looking for? or looking out for? Are there old units/new units, etc?
Make sure the software is compatible with your pc/operating system. I had to spend $100 or so to upgrade to their new software version to get mine to work with XP - still got a decent deal though.
Check for corrosion on the stand. Make sure that the unit has the latest firmware. Their Web site has the details. Other than that, they seem to hold up well. I bought mine used and have been enjoying it.
Good luck.