the only bad thing about buying off the internet is you dont know what you will get.(it could be a referb,missing parts,or not work at all.) i would buy from a local store,that way you can see what you are getting before you pay for it.
(you get what you pay for) JL 500/1 OR MTX 801D would work nice. do not worry about over powering your subs. the more power the better!! you can save money BUT you will not get the same quality.
it also runs more then the door chime most of the cars computer is set by the factory head unit. in some cars the air bag system is also tied into the factory head unit.(good luck with your unit)
have built many boxes for the L7's 8,10,12,15. never had trouble with port noise. the specs on kickers web site are dead nuts. (what type of wood are you building the box out of?)