I just fitted an Alpine 5952z to a 97 model 850 with an SC-801 radio. It works like a charm. Any M-bus Alpine CD changer should be fine. See http://www.alpine.de/2003/audio/datas/de/audio_chg_hu.htm and anything in columns 3 and 4 should work.

You may have seen that there are usually two CD changer adapters available for Volvo 850s, a cheaper one for pre-95 cars and a more expensive one for later models. I found that, although my car is a later one, it didn't need the more expensive adapter to get it to work as the radio itself is the same design as that used in the earlier cars. In fact, I sidestepped the whole issue (the adapters are ridiculously expensive here in the UK for no good reason) and fitted a full-size DIN 8-pin connector to flying leads that I soldered into the radio in place of the original CD connector, together with a permanent power connection. This allowed me to plug the CD changer cable straight in.