As far as the changer, there are a couple of Clarion models that are usable. The CDC-6x5 (x=0,2,3 that I know of) will also fit. I bought one off eBay a few months ago for $60 or so. I searched for Saab CD changer, as well as Clarion CD changer. I think the first part of the model number has to be CDC.
Fitting is easy, just plug it in. Locating it can be an issue. I didn't want it exposed in the trunk, and I didn't have the official Saab mounting cage. I did have brackets for it though, so I attached it to the underside of the trunk spare tire cover. There's enough slack in the cable to get there, and the changer I got (CDC-635) fits nicely inside the rim on the mini-spare.