So these are going to be slim subs?
Are you going with a prefabbed enclosure? If so that tells you your mounting depth limits. If not, have you measured, and planned your box yet?
Are you prepared to add a larger alternator? Could be $200-$600.
What is your budget?
So you haven't yet bought your amp, subs or enclosure yet? - If not, generally you can get more bang for your buck, trying to plan for 1 ohm. You wire them in parallel to bring down your ohm rating. Many subs have two options in ohm ratings. For 2 subs, you want either 4 ohm DVC, or 2 ohm SVC subs, and you can wire them down to 1 ohm.