Alpine F1? A lot!!! If you ever find one for sale!!!
But tbh, how much are you going to notice the accuracy or use the processing power, when listening in your E30???
These boys spend a lot of time and money sound proofing, building enclosures/mounts, etc, before putting equipment anywhere near the car!!!
You really need to start with a budget and work from there!!!
Balance it out with a bit of soundproofing behind the front speakers to help them out, then decent front speakers and head unit.
Depending how you do it, you can always upgrade as and when cash flow allows - amps, sub, rear speakers.... how serious do you wanna get??? EQ's, processors, line boosters...
How good are your ears??? Go round some shops and listen to demo cars at different levels, with different brands in them!!!
What music/sound you like can make a difference!!!
People like Becker h/u's (never tried them myself)...
£2500!!!! Clicky...
£500 Clicky...
Tbh, most people would be happy with some decent front speakers, a mid range h/u and a little sub.
It's a sensible starting point, you can amp the front speakers later, add some rears, upgrade things at will!!!