What you are trying to do? Turn your car into a club... lol
I think the secondary battery, new alternator is a little overkill. The stock battery (although I upgraded to the Optima Battery 8040-218 D35 YellowTop), and the stock alternator should be good enough for your need.
I don't know the spec of the alternator on top of my head, but I am pretty sure it can handle 100+ amps, I have a 100amp fuse inline with the battery going to my 4-ch amp and my mono amp.
My 4-ch amp is an infinity 1000W (400W RMS) 100W per channel, I also have a mono amp Pioneer Gm-D9601 2,400-Watt powering my sub ( also a Pioneer TS-SWX3002 12" 1,500 W MAX / 400 W RMS within an enclosure that sits in my trunk). If you are looking at RMS power, we are talking 66amp total. I have a voltage reader on my dash that is connected directly to my battery, so I can monitor it; it usually reads 14.4V. When I am playing music very loud, it usually drops to 13.8V, but I never saw it go lower. Which means the alternator is able to keep up with the power requirement of the amps. I have been running it for 2 years now, and sometimes I commute 3hrs listening to loud music, never saw a hiss or my lights flickering. My 4-ch amp gain is only 1/2 way though, so I am not really pushing it. The music gets really loud when I turn up the volume, I can feel the bass shaking the back window and my seat.
What I am trying to say is you might not need all that power, specially in a car environment, unless you are going to a car show and competing. I would concentrate on spending my money on your audio equipment, and then focus on the Bat/Alt if things do not work out.
Coaxial are great, I do not think you will noticed much difference between Coaxial and Component, specially if using an amp. Component are best though, specially in the front, because you will feel the sound surround you better, the murano has a good tweeter placement at ear level.
Coaxial in the front are easier, since all you have to do is disconnect the cable on the murano tweeter.
If you get component, then you will have to do more work to replace the tweeter and route the cables from your crossover to your new tweeters. It's all a matter of preference, but either one will work great I think.
The stock head unit is rated at 40W /ch @ 2ohm.
I can tell you for sure that the weakest link in the murano is the amp in the head unit, very weak sound processor. If you get an amp, it will for sure improve the sound, but you will be amplifying a sound that is already amplified by the head unit, so you might not get all the bells and whistle or a clean sound. You will have to experiment wit this, as I have never tried it. Some 4-chn amps have both high level inputs (speaker level) and pre-amp inputs, which means you can tap directly into the stock head unit without too much distortion.
I trashed my head unit all together the minute I bought the car.
Your speakers should fit deapth-wise, the door is pretty deep (hallow).
Since you are an audiophile, an amp is a must, you will never get the sound you are craving for without an external amp, the speakers will only come alive when powered with an ext amp, a Sub will definitely complement that. The murano trunk has ample space for a sub placement, so no issues there.
I would first replace all 4 speakers, and see the difference it makes, this might not improve the sound to the level you want because you are still dealing with the stock head unit.
Then add the 4ch amp and mono amp for the subs, this should make it louder, but might still be lacking sound quality.
If still not happy with the sound, then replace the head unit.
If you want to keep the head unit, then you can get and additional sound processor to improve the sound quality. I do not have any experience with that, but another member PaulDay has done something similar while keeping his head unit, he has the Bose though.
One thing I can tell you is that this is a lot of work if you want to do it right, so take your time.
If putting an external amp, you should get power directly from battery by routing your cable through the firewall. I had to learn the hard way when I tried to power my amp through the fuse block, although I was experimenting, I was blowing out fuses like pop corn.
No need for the "BIG 3 UPGRADE", the murano power/ground cable from the alternator are pretty thick and good quality.
GND to the amp can be connected to the car chassis.
Most of the audio stuff I did to my car, I documented here also. Through trial an error, I was able to get it to the point where I am satisfied with the results.
If you have access to the service manual, check out av.pdf where the audio system is documented in detail.
2015 Nissan Murano SV ( Custom MODS )
- Illuminated kick plate (non OEM added)
- RGB Foot well lightning (multiple colors)
- RGB compartment light (follows foot well lighning and color)
- LED DTRL on fog frame
- Cup Holder LED lightning
- Android head unit
- Custom leather seat cover (red accent)
- Custom floor mat
- superbright LED Dom light
- SGNL superbright 55W 6000K headlight
- Kenwood powered 150W subwoofer
- Kenwood 600W amp
-LED turn signal
-Accent LED lightning