2007 total after audio market replacement

I'm buying my first prius next week and I cannot wait! It's a 2007 with navigation and JBL with 6 cd changer.

I'm an audiophile at home and built my own DIY speakers that helped me fall deeply in love with great sounding music. I really want as similar a quality as reasonably possible in my car. I changed my wife's stereo and speakers and that's all I've ever done in car audio.

After poking around this website and reddit carAV, I'm looking to do sound deadening everywhere and replace the speakers, head unit, run new thicker wire everywhere, add a sub and amp by deleting the spare tire.

I have several questions, as I'm really new at this and a little scared.

Is running new wire actually necessary or am I waisting my time chasing diminishing returns.

What have you done with sound deadening and how successful was it?

How much power is it safe to draw. It seems like all the forums says nobody knows but there's got to be some opinions on whats safe and what's pushing it. I don't want to damage my car and I want to run two amps, one for speakers and one for sub.

Is adding a capacitor or second battery a bad idea, or good idea when you don't have an ac alternator.

How much does the draw of extra power affect MPG's because I assume it'll engage the engine more.

To people that did a total re-wire was it worth it? How much harder do you think it was not being able to just hook up to the stock wiring harness.