Hey Felix;
First thinG I'd consider is to NOT replace the rear speakers unless you feel the need to spend money. Remember that rear speakers in the car are for ambiance only.
In my Civic(still stock execpt HU) the rears are probably 12Db down compared to my fronts.
I always ask people(if they wonder why my soundstage is all up front) which way they look when they watch a concert?? Do they turn their backs to the concert??
My first suggestion is to not get a flashy HU. Besides the fact that people love to steal your car(same with me

) you pay for pretty!
I use Sony, and although some people say "stear clear" I've never had any problems!
Within reason I'd get anything you like with the features you like ..... WMV, CD-RW, MP-3 ???? I WOULD stick with the bigger names because they are that much easier to find service.
Pioneer(faceplates are to busy for me), Alpine(pay for the name), and Jvc, Sony, Clarion are all good.
Eclipse WAS a good name, not to sure anymore.
Some like
this
Seems solid. I like the fact that it includes a parametric EQ, and 3 sets of preouts ...... NO need for and outboard crossover
As for amps, I'd stick with a four channel, use 2 for the fronts and bridge 2 for the sub, use the HU to power the rears.
I'd look at something like
this . Seems like a heck of a deal!!
As for speakers, you have 2 tweeter upstairs right?? So go for a component set. Although that makes the multiple crossovers redundant that cool
Something like;
low end
this
mid price
this
or high end
this
These are just ideas taken from 1 web site ... Although I do think the Xtant deal is great!
The real question is how much room do you want to take up with a sub???
I'd look at something like
this
if you want something that's easy to remove.
something like
this is also an interesing option. I've never had a problem with my Pioneer 12" in a 2 cube sealed enclosure ..... and it's cheap and !!
For sub, I'd get a nice 12" in a small sealed enclosure, it'll go low and loud .... nice to have a hatch for that reason
On the
lowend
I've also heard a lot about Audiobahn, but I know nothing since they weren't around when I was really into car audio, but I think you'd be looking for something that doesn't fill up your hatch!
WooHoo
Brent