Looking to upgrade original speakers

[dtf]

I posted this on the Car Audio site but go no hits. I'll try it here too.

I am almost totally clueless when talking about stereo components. I am looking to replace the stock speakers in my '94 E320 wagon. I think they are 5"(?) Speakers. I would like to change the front dash, lower front door and rear door speakers. I currently have the stock radio (what kind is it - a Becker?). Can someone give me a recommendation as to what brand speaker to put in and where to get it? I also have future plans to finaly put in the CD changer. I've heard that putting in an Alpine is cheaper than what MB puts in. Is this true?