At the very least, replace the head unit at the same time as the component speakers up front. And you are right, no need to spend the extra $ for components in the rear, IMHO. So I would go with just typical 2-ways.
You can always amp the speakers down the road, at least the fronts, and like you said, if you feel you need more volume (which is likely). I am fixing to seal behind the front door panel trim, and I am using Parts Express's adhesive-backed vinyl sound-deadening sheets. I would try to do something similar, and any similar peel-n-stick for roofing / wall material product from a chain hardware store will help.
So back to your question about which Focal components you should get: I cannot help you there, as I just stick to Polk dBs. Sorry. But I would try to get the largest diameter you can fit, and do what ever cutting/trimming/custom mount plate it takes. Just verify mounting depth, so the window doesn't get stuck on it. Also I will be having to cut the vertical additional "supports" (behind the horizontals of the face) in the stock door panel (2002), as with the 3/8" plexiglass mount plate, the surround appears to hit it some.
A couple of specs that I would recommend you look for in components:
1) Rubber Surround - A MUST; which I will assume all Focal components are...
2) Watts RMS - And if this is higher, such as 50+ watts, then an amplifier is highly recommended. (I'm just taking a stab in the dark with the 50+ guesstimate, I have not been in the market, and cannot recall).
3) Sensitivity: I think of this as how efficiently does it turn power into music; not sure if that is exact. But look for greater than 85, the "better" ones are 90-95+
4) Frequency Range: How low can it go? Generally larger diameter will go in to the lower Hz. This is not as important if you are adding a sub later. But sealing the door "plane", or even better would be to create a custom MDF board / fiberglass enclosure, will greatly help with the lower Hz.
The high-end of the range frequencies are what hurts my ears. The Polks go up 23,000, and other components typically will go higher than that! As long as it is at or above 20,000 then you are good, just turn the treble down
I would make sure to get the right components up front, and dont go to far overboard on their expense. With what you are describing as your needs, I would be looking in the under ~$200 range, but I am just throwing that out there. I dont think you have the need to get anything too much better than Polk dB 6-1/2"/6-3/4" components, but again, IMHO. I dont work for Polk; promise. And get a decent head unit, with whatever features you want: usb, blue-tooth, iPod interface, back-up cam capability, etc. Important specs to look for are # of RCAs out for your future amps, RMS again - if there is a possibility that you will not be amping some of the speakers, and S/N ratio. Look for it to be at or above 100. Last I checked, Alpines were always way on up there on those items, even their entry models. Check out Eclipses as well.
And good luck! Keep us updated on what you decide and end up with.