The cap doesnt do anything. Get the big 3 upgrade and a new battery and you will be good. The only thing a cap is really good for is looks in a show install
well, its a good rule to go by with subs which draw 1000wrms. it can help with headlight dimming every time the sub hits. not "necessary", but good to have.
You could always just sound deaden really well. That is recommended with any high end speaker anyways. I like dls personally. You could go with up6 or ur6 in front and 226 coaxials in back or just stick with what I recommended in the beggining
I have never heard them personally but dynaudio has a good repution. You prbally wouldnt regret it if you did get those. I take it you still plan on building kicks for that 7" mid?