Hi! Given a good enough deal on a working arm, it could be worth a try. I think I'd think about the following stuff (in no particular order)...
1. Is the arm interfacable to a PC so I can control it? If not, can I realistically hack it?
2. Is it going to be able to move and meet the tolerances for what I want to make?
3. Is it powerful enough to handle the milling end effector I want to use (spindle,router, etc)?
4. Can I find CNC software that works in spherical or cylindrical coordinates?
5. Will it fit into the space I have, even as the arm moves through it travel extremes?
If I could answer YES to the above and the price was right (including shipping), then I'd go for it.