Try Plug n Play system so it does not void your warranty. Like Match, Focal and Rainbow

Perhaps can check fb page https://www.facebook.com/profile.php?id=100018664238418 for pricing..
If warranty is not an issue then you can visit any reputable shop to change the entire system..

1. Depends on you. I would say few things at 1 go . Head unit -> 2chnl amp -> front component speaker -> monoblock -> subwoofer. If you want your rear passenger to enjoy only you buy 4 chnl and rear component speaker

2. If you are listening to edm, then dont buy underseat sub. It is a complete waste of money especially when you got bootspace and willing to sacrifice some of it. Im aware that hybrid version of your car actually has quite a bit less room for the boot. Buying a proper sized subwoofer box is crucial for sufficient punch to your low-end.

I assume you subscribed Spotify Pro as your source but i can tell you, with such system you should definite look out for .flac files instead. Theres a difference in sound quality.

my suggestion. go for EA autoworks. never regretted

For things like this, easier to just go the shop and discuss in person.

But if you want my opinion, I would suggest the following.

1) good headunit
2) dsp with built in amp
3) underseat subwoofer
The above 3 can be done in one go but may go a littler over your budget. In the future, get better speakers and soundproof your doors and maybe an amp to drive your speakers.

Underseat sub is more than sufficient for normal listening. If you listen to really hard edm/house/techno/bass songs and want to make everything in your car vibrate, then go for a proper sub and place it in your trunk.
Currently, I have a kenwood single din, crossfire dsp, underseag sub and jbl speakers. Its enough for my listening.  I'm using the cheapest underseat sub in the market and it's enough to make my rear view mirror vibrate 😂