I had a CD700 for a while and there is still a TD1200 in the 323 (form the days of tapes

), It stood for 20 years and fired straight up. They were never the cheapest thing but the price they fetch now is stupid, I will be taking it all out carefully!
You can get good speakers to fit the standard mounting holes behind the footwell grills easy enough.
There are tweeters that fit in the OEM door mounts, some easier than others Depends on whether you want to butcher a pair of original ones or make your own.
The problem with 6x9s in the rear is you will not only have to cut the cardboard parcel shelf but the metal underneath, unless you use something like the BMW 'premium speaker' which sits more 'above' the shelf.
I know this because I cut the shelf in the chromie to fit 6x9 speakers for the Nakamichi and am now having to weld it back up

You may be forced to have a bass box rattling around in your boot if you don't want to cut metal.
You will need an amp to run any good speaker (and I think the Nakamichi only puts out line level signal anyway), a crossover to push the right frequency to the right speaker and make up for the bad acoustics of a car. All easily hidden
It is over 25 years since I put the system in the chromie but more recently I have used Focal speakers which I like and will probably use them to replace the old B&W MASS system in it now.
If you are on the dreaded zone facebook Kamal Sokhi is worth getting in touch with as he has done a lot of E30s and has a bit more up to date knowledge of what is about that works.