hey. addressing your second question, the bass blockers should work just fine. just wire them in line w. your speakers and you will be good. is everything in her car stock right now? if it is and you are just looking for a sub to carry out the low end id suggest like a Sony or Pioneer Sub. they are decent subs that will support some good low end bass without spending too much money and without getting loud pounding bass. if this is going in an all stock system car, i would personally power it with between 150-250 watts rms. if its aftermarket anything 300, 350 watts rms and above will be good.
Sony Sub 12"

Pioneer 12" Sub