An infinite baffle setup would require you to completely isolate the trunk from the cabin of the car. You would need to build a baffle that the subs attach to 1-3" thick. That would be the seperation point. You can face the subs forward, rearward or vent them through the rear deck as long as you have a good seal. I think you may gain a extra db or two from the rearward firing. Building a normal box would be much easier and less time consuminmg.
You'll need to reinforce your trunk. Think of it as walls of your box and you want them as rigid as possible to gain the most db's out of it. You will need less power as well as most IB setups run 50-100% of your rms to avoid the power bottoming out the suspension. (this varies with each driver) IB setups have a certain peak in frequency. You can use an subsonic filter or parametric eq to tweak it. You also won't see much output over 80hz. They have a much different sound from what I hear, much more musical but that is anecdotal at best.
I have just started my IB build with two 18" Dayton dcs-450's. Ill be giving each of them only 240 watts a piece. Shy by many standards but we are talking about 18" drivers.
There are alot of talented cats on this site that have the ability to put ten lbs of stuff in a five lb bag. Make a new thread and I'm sure they can give you a handful of ideas to get you started