You don't need a wet dry. The protein skimmer will work fine on the back of the tank (as long as it is a hang on back skimmer).
You will still need powerheads, because that won't be enough flow. I'd recommend at least 4 powerheads for a 120 gallon tank.
What kind of skimmer did you get? Some brands are better than others. Try to stay away from the SeaClone and Red Sea Prizm. They don't work well.
Also, with a canister filter you'll have to be very diligent about cleaning it, or else you may see nitrate and algae problems. Canister filters usually need to be thoroughly cleaned at least once a week.
You may want to think about adding a sump to your setup. That way you can hide all the equipment in it (less clutter in your tank) and add water volume to your system.