In terms of the dash speakers, the size is strange, and there are only a limited number of models that will fit. Some folks have performed creative surgery. Others have put them in the doors instead. I have not touched mine yet. Do a seach in Car Audio on "Front Speakers". Lots of info.
In terms of a CD changer, I suggest getting a CD player that supports MP3 playback instead. You can rip 10+ regular CDs onto a single CD in MP3 format, and delete the tracks you do not like in the process. Cheaper hardware, and less baggage to carry around.
1986 300E 5-Speed 240k mi.