Why not pick out a new HU that has all the bells and whistles you need, want, and for later use?
Just a suggestion....
Since your undecided, and nobody can know your personal preference but you, I suggest getting a unit with 2 RCA amp output jacks. 1 amp for the sub, and one for the door amp. Buy a sub amp only, and run it. Then decide if you don't like it or not. If not, add a 4 ch amp later for the doors. More work, but not that bad really. You can set a 2 ch amp to mono for a single sub, or use both ch for 2 subs. Amp should come with the 2ch/mono switch.
A good set of door speakers will make a huge difference in sound, with or without an amp. They can sound great using the HU power supply only, and today's HU supply is typically sufficient for mids and highs. Some sound better with more power, some don't. Minimal THD on both the HU & amps are important for clean sound, the lower the better. Don't go by the high wattage they boast, as that is just peak watts. Real watts are much, much lower.
I run a sub amp only in the DD, but I have changed the door speakers to RF component 6.25" round, and separate tweets using adapter plates. These make more mid range bass then the 5x7's do. They can also take more power if I ever decide to add another amp for them. I like how it is now, and am very happy with it as is. But... this is my personal preference, not yours.
To me, sound quality is way more important then how loud it goes. I could care less if the people on the next block can hear it. That's actually very annoying. Having so much bass that it drowns everything else out is also annoying. Balancing it all out is where it's at for me. My Dbl Din HU can set the distance from my ears to each speaker in inches, and time each speaker to fire so I hear them all in unison. I want to hear that drumstick tap the symbol as much as the bass drum getting hammered.
Get rid of all that factory wiring, including the speaker wires. Replacing 25 year old factory wires with new shielded, better gauge wires with no breaks, splices, etc.. will make a big difference. Take your time with it, and enjoy doing it. If you have to rush any part of this work, shortcuts will happen, and quality will suffer as a result.