You need to know is what you'll be using the impact wrench for most of the time. This means what size bolts, and what the manufacturers recommended torque is for those bolts. From here you can figure out what size drive you need by knowing what sized sockets you need (you'll want to get impact sockets as well, regular sockets can shatter when you use an impact wrench).

You need to ask yourself is where you'll be doing the work. Will you, or can you be near an air compressor? Air wrenches are typically lighter, cheaper and more reliable than their cordless equivalent, but if you're not going to be anywhere near an air compressor, you'll have to go with cordless (cordless technology has been getting much better, but I do have a soft spot for air tools). This is the first thing you need to know.

With the recommended torque, you will want to add an acceptable safety factor to figure out how much torque you will need. If you get an air wrench, typically they will give you a max torque, and sometimes you will get a working torque range as well.

First, you will want to add a safety factor on top of the manufacturer's suggested torque, to account for the environment that your bolts will be in (I'd recommend you take a torque wrench out to see how much torque you need to loosen a "worst case scenario" bolt to get a realistic number). You'll want your impact wrench to be able to easily handle that "worst case scenario" (no one wants to bring out a breaker bar because their brand new torque wrench isn't powerful enough), so make sure that this is easily within the "working torque" of your wrench.