Advice on Subwoofer install for X5

[cooa99]

Hi, I am about to build a sub box to fit into the rear compartment of the E53 (under the fuse box). my questions are:-
1) Should the box be ported or sealed
2) should the box (after beign carpeted on the outside) be a tight fit or should there be enough space to add paddings (& dynamat) to dampen vibrations
3) what subs should i consider for tight base response?
4) my x5 does not have an amp, If I tap into rear speaker wires to feed a subwoofer amp with Line out converter, are there going to be issues?
5) can I get away with a 6/7 channel amp to power the subwoofer and then my door speakers(in the future)?

Thanks
cooa99