With the cd player, I think the changer is meant to be built into the headunit or something, although I havent tried it and I cant find anything to explain it. Your mechanics point about ride quality is pretty fair, although I find mine has quite a harsh ride anyway - although whenever I do actually do something, I'll probably do the boring/sensible thing of normal rubber bushings. New plugs is interesting too, mines got 102k on it and I doubt theyve been changed any time recently (hell they might even be factory original).
Anyway, with getting something with bluetooth, a somewhat popular swap is to put the unit from (I think) a newer K12, or a note. However, as your one will have the same as mine, you have ISO connectors as standard, so if you dont mind losing the oem look (and the ability to control it from the steering wheel) you can quite easily put in any standard double din headunit, the most work it could possibly need is an adapter cable from ISO to a specific connector for certain brands
