1) yes, you need the correct harness for it. PAC sells quite a few, not sure which one will work though.
2) Should be plug and play. Monsoon system probably has built in EQ for the stock speakers so it might not sound all that great unless you bypass that system completely.
Speakers come in both coax and component. Up to you as to what you're up to installation wise.
3) Sealed subs sound great granted the box size is correct. My 18 is quite nice sealed. Been in a GP with a 15 ported in a small box, was not balanced at all.
One sealed 12 should be enough if you're keeping things mostly stock.
