My advice is to get something with a Fanuc controller, or "Fanuc style" as the programming and coding will be MUCH easier.
When buying used CNCs,:
1)See it run! If not running and not under power, beware. It could be a brain dead boat anchor that will be a real PROJECT to get running again.
2)Make sure manufacturer still supports with parts and tech. Many Taiwan builders go under, only to re-surface and when they do, the previous machines no longer
have any tech support.
3)Make sure it has manuals/schematics/parts/ladder books. There is an EXTENSIVE amount of literature that comes with a CNC and much of it gets LOST over the years. Without it, you could literally spend as much on the books (if even still available) as you did on the machine.
4)Avoid orphan machines/controls. Stuff that isn't made anymore. It could be a NIGHTMARE for parts.