Some good advice here, but I just wanted to make some further observations:
1. To be able to take advantage of an external DAC, be it a seperate device or a DAC in your AV receiver - a CDP needs to have a digital output. That's not a given with older CDPs.
2. If you use the digital output, obviously the DAC in the CDP as well at the output amp are not used. However, if you do use the analog output, the output amp can make a huge difference - in my experience as big or bigger than the DAC! The good news is that output amps cost peanuts. If you are handy with a solder iron (I am not) or have access to a tech who is, swapping out the output amp can make a dramatic difference to the sound of a CDP. I have an Onkyo DX-702 in which dr*audio changed the output amp, and the difference in sound was like night and day.
3. I understand that the laser assemblies for some older CDPs are still very readily available. For example I believe my Onkyo DX-702 uses a Sony laser that is both cheap and widely used. The mechanical part of the transport mechanism, like all mechanical machines, enjoys sensible lubrication once in a while. Obviously you don't want to use too much, or get it on the laser!