Yeah, you should either get 2 or 4 subs to keep the wiring right. Unless it's like some of them out there that make 12 ohms so you can run 3 subs (kicker doesn't). Decide on how many subs you want first, then start thinking about the wiring (I.E. hooking 2 up to one amp and running another amp for the rest) and then you will figure out how many ohms for the subs.
As for the placement of the 18's. I can't do them in the hatch of course (can hardly fit a 2 12's, you could squeeze a 15' there if you really try hard). There going in the back seat behide the roll bar. Then they will installed angled back so they can echo off the hatch glass. My 2 JBL 1200.1s come tuesday, then I just need to get another battery and minor stuff then take it to have the box made. Shop is doing the box for me for 125 bucks (supplies, install, carpet) I didn't want to do it cause I've never built a box before and it has to be put together while in the car pretty much to get it to fit in there.