Why I stated what I did is this. The link you gave are to home designed drivers. The sound from them is meant to be heard directly. Speakers mounted in your doors fire straight into your legs. Car speakers are designed to have a better frequency response off-axis. There is a definite difference in the way speakers are made.
Another small problem is these home drivers are 8 ohm. I.e. if you car amp is putting out 100 watts at 4 ohms, it will only put out 50 watts into the 8 ohms. That is not necessarly a bad thing (amp will stay cooler, last longer) I thought you might want to take that into consideration though.
Sorry, I have no ideas on VrumVrum's prices. Give him a PM, he'll let you know.
Best of luck to ya