I never used to think the under seat subs were worth it until I experienced one daily. Solid 8-9/10. The one in my car is a sony, probably 5 years old. Has line control as well as the independant controls from the H/U. Adjustability is key with these.
Sony mids in the front doors, added sony tweeters to where the door on the inside of the mirror and rears and left stock. Both fronts are amped from an ott sony unit.
Your head unit will come down what you want out of it. Single or double din. Amount of line level outputs. Amount of RCA outs. Aux front or rear, bluetooth, CD player, DAB, ect. The one I have is an older sony xplod unit. At the time it was the only hu with dual rca out, 4-6x line level, sub out, rear aux in, CD player, digital FM, a full equalizer and graphic display.
A lot of new cars rely on lots of average spec speakers driven by kick ass amps and head units rather than quality speakers, so thats something you could do too.
My pick, would be the sub, going to 6” fronts apline for mids, alpine tweeters in the front, and adding in a set of rear speakers, anything will do to start with. H/U will be your choice, best to speak to an audio shop to get yourself up to date on all the models ect. Same deal with amps. Any name brand will be fine.
I would do the fronts, sub and h/u, then get used to it all and see where it lacks. Upgrade from there. You may find it lacks depth or volume, so do the amp. You may find the lows are all coming from the sub, so add some rears to do that job. Ect ect