Infinities are a good choice and the Dynamat price you found is cheap.
The tweeter will replace the stock one, there is room behind the plastic cover at the door. Make sure they point between the front seat's head rests
when you fit them. This is important.
The box thing (crossover network, or simply crossover, or xover) is to devide the signal in two parts, one for the woofer and one for the tweeter.
It will be placed inside the door trim (usually there is space under the arm rest area.
About the rear doors, you could get something like this ( supposing it has 165mm speakers )
If you decide to fit them yourself, you must have in mind some things:
1) when applying the dynamat, the goal is to isolate any radiation ( sound ) that comes from the rear part of the woofer, to leak to the front side,
in other words, there must not be any opening anywhere at the door of which the rear sound can come outside and reach your ears. Rear radiation of the cone
has 180° phase difference, canceling the front radiation ( mostly on low frequencies.)
2) The tweeters must point at the listeners' ears. If their distance from the listener's ears is smaller than the distance between the woofer and the listener's ears
you must use the -3dB or the -6dB option of your crossover. This is to have balanced frequency response.
3) Watch the polarities. Connect the + to the + and the - to the -. That is strict for both the woofers. Connect the tweeters like that too. If you find the soundstage
funny, or your mids a bit messy, try inversing the polarities of both the tweeters. This is to deal with phase problems ( due to the different distances).
When testing the installation, use a cd with natural instruments and clear vocals.
If you let a professional to do it for you, make sure he follows the above rules if you can be present during the installation, the owner is always more carefull on details