You have to select the player yourself from you're needs, I would have interest for the CDC-610 because it was the first or maybe the CDM-900 because it is crazy (it holds 110 CD's). Other than that I would not get a changer.
If you look a bit closer to the changer line, then you see it was not especially made with high-end stereo in mind it is more about convince. But you do find ok DAC's in some of them, only that is just a part of what makes the sound. This is good and bad; god because you can upgrade the parts and get better sound, bad because well... they are not build like the TOTL CD-P's.
Sorry if it sounds like I am trying to talk you out of it, you may have a god reason to get one (like not change the CD all the time

).
OK and now you have to get one of these things; I think there are 2 things to look out for;
1. The changer mechanism, how well is it working and how old is it (how much wear and tear). A changer that do not change is kind of worthless, so newer/less hours = better.
2. The DAC, there are a few different to look out for. I know Yamaha1 swears to the S-Bit plus (witch would be the CDC-635/645), he does not like the later DAC's since he finds they "change" the sound to much. Witch brings us to the PRO-Bit DAC (YAC-514F) it came in 1994 but didn't show in a changer until the CDC-765 (1997) witch in my opinion would be the first "best sounding" Yamaha changer, the PRO-Bit DAC is still used in to days CD-P's and changers.
So to stick with the 2 "rules" I would go for the newest PRO-Bit changer, the last one I have found branded with PRO-Bit is the CDC-775 witch sold from 1999 to 2001. The later models CDC-685 (2001-2006) and CDC-697 (2006- ) are not branded with PRO-Bit and have lesser specs, but it would be interesting to see what chip is inside (my guess is the YAC-514F).
I hope this helps you a bit
PS. Be aware that the model numbers are European numbers! We didn’t have the CDC-905, but all the mentioned changer manuals are on Yamahas site:
http://www.yamaha.co.jp/manual/english/result.php?div_code=
----EDIT----
AROW 68 : Yes the CDC-845 sounds like it is the US counterpart to the EU 775, it even looks better and has slightly better numbers.