Actually, replacing the deck is quite easy! I just supervised a friend who had never done it on a 91, and it took us about two hours, the two hours being watching TV half the time to just sit around and splice wires. The connection is quite straigt forward.
In regards to your options:
1) That's what I do, so far, because I like to change CDs on the fly and a 12 disk doesn't hold enough for me since I like to change them anyway. I don't like fiddling with the cartridges, but I'll prolly get one too someday.
2) FM-Style changers work pretty well, possibly better than what we have now, but you're not getting all from the CD changer, because it's going through the antennae. It's only as good as the antennae system, and you're losing frequencies through the antennae, though it may not be totally noticable.
3) Okay as long as you want to lose the tape.
4) Okay, but dual media players are VERY expensive, since it'll be compact, and it's only single din.
Check around first here:
www.go2pac.com www.pie.net www.soundgate.com
and especially this one:
www.radiosandmore.com
Radios and more sells OEM equipment at considerable less pricing, and maybe you can just get a factory one, and no other hassles.
One or some of those other three above sell the adapaters for between the radios and the changers, I forget. But one of those will be able to help you.
If I were you, I would REALLY go into just buying new speakers first. There may be no distortion, but the car is like ten years old, the cones have probably had it. Anyway, you can't get better sound if your speakers are still old. You might have a better sound signal, but the speakers are what will ruin it. Speakers will be a cheaper, and MUCH more noticable upgrade. New speakers will also have better highs, which are paramount to CD sound.
Good luck!
Jon