if you use a plastic bracket, it help to use a piece of soft rubber attached to the door panels. this is to avoid rattling which you will lose some mid bass. You can custom DIY it from a 3/4 plywood which i did. just make sure that your speaker's magnet does not touched the body of the car or any metal surface (if you dont want a fried speakers ,HU and amp with it). The big thing is not installing the speakers but the door cover. Those speakers would not fit to your door cover so you have to cut it to fit. (make sure you cut it a perfect round unlike me who have an egg shape hole on my door).
As for soundproof, if you find your door rattles and producing noise or the sound from outside is louder then the inside of the car then you should start sound proofing.
As for soundproof, if you find your door rattles and producing noise or the sound from outside is louder then the inside of the car then you should start sound proofing.