Basically everything mechanical fits. If you swap EVERYTHING, most of it will be a direct bolt-on. You will have minor issues with suspension (positive camber, ride height), steering components (u-joint will need modifications), and possibly the gas pedal. Besides that, everything else that matters literally just bolts on. If you need to mix and match parts from different cars, though, you will run into more issues.
Having said that, the mechanical part of the swap is by far the easiest. It requires very little skill and knowledge. Wiring will be MUCH more difficult, ESPECIALLY if you intend on using an sti ecu and oem DCCD controller. If you are determined to do this, figure out exactly what you want to do and what kind of wiring is involved first. If you figure that out, everything else is downhill from there. I suggest that you use a wrx ECU and aftermarket DCCD controller instead and forget about AVCS and DBW. If you REALLY want AVCS, use a jdm ECU.
There are aftermarket hood scoops available. Don't force yourself to go front-mount yet until you've gotten all of the important stuff sorted out.