I would suggest buying an inexpensive passive, amplified component set to replace your front stage completely.
If you're buying a set of speakers just for the midbass, it's common that they are heavier than stock and will require more power to move. So you're going to want to amp them.
Unless you have passengers in the back seat often, I wouldn't bother replacing the rear speakers. The gain in sound quality is pretty marginal compaired to the cost.
You will have much better highs, bass and midbass for two reasons: 1) the corssover filtering the signals better and 2) the amplification providing the speakers with more power.
Personally, I would sound deaden the doors and seal them. I would probably stay with the stock size unless you can find a high quality, inexpensive 6.5" comp set. Image Dynamics makes a really well reviewed passive 6.5 comp set.