I would look at where you can install speakers and amps first before you pick up any equipment. Make sure amps you pick out will fit somewhere they can breathe. Any good speakers can sound bad if installed wrong.
How does your system sound now, image? Is it possible to just upgrade present speakers? I like 8 inch door mounted speakers unfortunately most stock ones are low powered junk. I would look into an upgrade there with sound deadening to keep doors from rattling.(shallow mount 8 inch) If you don't mind cutting maybe larger speakers to replace the 3 inch. This may not be needed with good shallow mount 8's in the doors, just an upgrade for the 3 inch.
I would go to some car stereo forums and come up with an install plan first then pick out equipment. Yes it's work but will save you headache's in the long run and your system will most likely sound much better.
I would not install a box speaker unless in a van or SUV. Taking speakers out of box it's design is not good. Installing speaker from box would also be hard to mount.
Also I would not run tweeters or coaxial speakers in the rear for fill. Mid range is ok if not to loud, tweeters in the rear tend to bring sound stage towards back and ruin imaging.
1ohm load most likely designed into crossovers actual speakers are most likely 4 or 8 ohm.
Take your time, plan first and then install and you should come up with and good system. Cls can be your friend if you don't mind used.