Well about the sub, you will need to be able to run a wire from the eng (specifically near the battery area through into the cabin and to the amp). Then you need to run another cable from the deck to the amp aswell.. or atleast that's what I've read about.. heh.. and simply after that, connect the ground to a place where it'll be grounded well and hook up the sub to the amp.
For the subs, I recommend either 2 10'' or 1 12'' in a sealed box (if you love bass you may want to go with 2 12's or maybe 2 15'' if your really crazy :evil: . depending on what you want with your set up. Assuming that you won't be amping your speakers, I'd recommend you go with a 2 chan amp with a lot of juice

. If you are going to get it professionally installed, it'll cost about 60 bucks install for the amp.. And don't forget to get a good gauge kit..
As for the custom door panel, I have no idea how to do it.. lol..