Some people have made a good soundstage without dedicated subwoofer but it will involve some extreme custom work and typically require 8" full ranges up front. It will almost always be simpler and cheaper to add just about anything for a sub to fill in that bandwidth.
Building studio quality sound into the rather horrible acoustic environment of a car can be a costly, time consuming, and maddening undertaking. If you don't want to turn this into a serious hobby you should lower your expectations from the start. There's just too much stuff wrong with the "room" (space you're trying to fill with sound) for high quality sound to be easy. That said, reasonably flat from 20-20hz at 120dB shouldn't be terribly expensive, shouldn't require big electrical upgrades, nor any unusual amount of space.
Pioneer makes as good a head unit as anybody when you compare to similar priced offerings among the other reputable brands, mostly the difference in those is just user interface which you may get used to or never like at all; sadly there's no way to know what you will and won't get into before you actually try to use it.
Lastly I'd say that pretty much any outboard amp will be an improvement over whatever power your source unit is driving, and even 50-60Wx4 + 300Wx1 into a sub should get you a big upgrade. Not sure who all makes good quality fairly priced 5 channels these days but you don't need huge power numbers to have plenty of output and you should avoid brands with over-inflated power ratings on general principal since companies who come out the gate lying to you typically aren't selling good quality products.