So i don't know if these questions have been asked yet, but i didn't feel like reading through a shit ton of mud.
but anyways to the questions.
What is the trunk space situation?
kinda obvious, if you can't fit the subs back there, why bother.
What kind of box do you want?
there are a couple types of boxes out there.
sealed-cheapest, used for tight accurate bass
ported-more expensive, deeper bass
bandpass-most expensive, combination of the two mentioned before
What kind of music will you be listening to?
this can also affect the box you get.
personally, i like metal/rock from a sealed box, and dubstep, and rap from a ported.
I haven't had a bandpass box, so i'm not going to talk outta my ass about them.
Have you replaced your head unit yet?
it'll be a pain in the ass if you have to wire an amp to a stock headunit (i'm pretty sure they don't have rca outputs).
Is money an issue?
you can put a cheap system in for 200, or go all out and spend a grand.
will you be installing this, or professionally done?
if this is your first venture into car sound, i would suggest either getting it professionally done, or with someone who knows what they are doing.