I went a bit OTT on mine.
Alpine head unit.
Amp in the back, under the ‘overhang’ being the seat at the bottom of the bulkhead.
Speakers on custom made plywood baffles, door cards trimmed to clear and drilled out the little ‘dots’ on the tweeter grill. I used 5 1/4 speakers but think 6 1/2’s could fit.
Fully deadened the doors, outer skin and inner, as well as sealing them to make the doors into an enclosure.
Sub.
Sounds excellent, but tweeters in the A pillars would be better.
The sound alignment thing on Alpine, just using the phone app, made a good improvement on the stereo image as speakers are placed wide and low.
How far do you want to go and how much you got? I did mine for about £600 but that was DIY with a big discount on the Vibe amp, deal on Hertz speakers and 2nd hand sub.
I’d say start with fitting an amp, then add well installed door speakers and damping.