And then look for some 6.5 inch coaxial speakers to fill the hole. Personally i'd look past Pioneer and look at something like Focal, Hertz, Ground Zero, JL Audio. All can be had for about the same about as Pioneer speakers but will be much better. If you're going to add just coaxial speakers make sure you disconnect the factory dash mounted tweeters as well.
Really tiny and light amps but have a huge punch to them, and they can really be mounted anywhere in the car, Then i'd look at a decent set of speakers again anything by Focal, JBL, Hertz, JL Audio, etc. Just make sure the speakers you buy don't have massive magnets because the opening in the door isn't massive and you'll have to cut the door skin to make it fit and thats a pain in the bum
It all depends on your budget. If you've got less than Â£150 to spend i'd recommend getting the speaker adaptors and some half decent coax speakers and then disconnecting the factory dash tweeters, but if you want the best sound I'd get the adapters, the small amp and then some decent component speakers. It's not a massive job but if you're not handy with spanners get your local car audio place (not Halfords, a proper car audio place) to get you a quote, shouldn't really be more than a few hours work for them
I used to have 6.5'' Focal polyglass in the doors with spacers, Focal tweets in the dash and 12" JL sub & amp neatly in the back, Alpine active head unit, so no need for crossovers. Just had it as a 3 way system didn't bother with speakers in the rear.
Had this in both my previous DC5's, sound quality was amazing especially considering they don't have much sound deadening. Best use some Dynamat or similar in the doors though.