First you need to set a budget for what you want now.
I'd suggest you'll need.
Sub, amp, sound deadening, cables, Fiberglass, mdf.
Figure out a budget then you can figure out how much the cables, fg, mdf, sound deadening will be. This will give you what's left for sub and amp.
Not a lot at a guess so drop the deadening for now to free up a bit more cash.
Have a look at what subs you like the look of. I say this a lot bit get yourself over on Talkaudio. Make a few posts so you can view the for sale threads.
There are loads of bargains to be had. You'll get ten times the equipment if you by second hand. You'll also see what other people are running and there thoughts.
For me JBL are king budget gear. Ive run various setups with them. One was a single 12 inch GT in just over 1cu ft sealed. Sounded lush.
Disconnect the tweeters, ignore rear speakers and put your effort where it matters most. Try fixing your fronts to the door skin, not the card. Use as large a piece of mdf as you can fit behind the door card. The stronger this is the better the sound will be
I'm going to be making an entire door card out of birch ply to get mine as strong as possible.