Make sure whatever you buy has GOOD manuals! The first machine I got, a Philips-brand CSM made by Yamaha, had quite good manuals. The second machine, the Quad-brand made by Samsung, had absolutely the worst manuals I have ever seen for something so complex. Fortunately, somebody hooked me up with the guy who did the factory training on it. I mean the most BASIC stuff was not in the manual. For instance, the machine coordinates are obvious, +X to the right, +Y away from you as you stand at the front. BUT, the PC board coordinates are +X to the LEFT! This actually makes a bit of sense, as the corner that is fixed on the conveyor is the front right corner, so all coords grow away from that. And, maybe this coordinate system is configurable in the ini file, so they can't say which system has been configured. But, this REALLY had me going in circles. The factory training guy was a godsend. He was a bit rusty on some of the details, but I sent him screenshots and he was able to explain a lot of stuff.
One other thing, if you are going to get an older machine, make sure it was popular at one time. My Quad/Samsung machine is based on the Samsung CP30, which was very popular in the far East at one time, and so lots of spare parts are available.
Jon