SykSVT, I haven't been on this site much, but can't resist helping out a fellow rally fan! I run a 2.3L focus in NASA & RA Rally events. I threw out the option of building for the spec focus, one because I had the wrong motor, and two because I had to buy expensive contingency sponsor products, like DMS suspension...but the spec focus website hasn't been updated in a while, so maybe it's not even running. Anyway, I wanted to help answer some of your questions:
Why 15" wheels? 15" rims are easy to come by for the Focus, but more importantly, anything bigger than 15" rally tires gets really expensive and is not readily available. 14" and 13" wheels would also be options for you though, but I wouldn't recommend that.
You wonder what's the best upgrades to do next...I'll echo other people: Suspension and traction hands down. First your car has to survive an event. I've used stock suspension and KYB AGX's, both failed by bending severely right at the Knuckle. Ended up with Hotbits. 2.5 years ago I paid $1500 for entry level rally Hotbits suspension and they've never failed me yet. Also, get a loose surface LSD. By this I mean clutch type or anything else that still works when one wheel has no traction. I have a Quaife in right now that I'm about to sell because it's not good for loose surfaces. I've been stuck in a ditch because one wheel was in the air. It sucks.
The minimum you have to do to your car to rally is as follows (prices are based on my own experience):
-Cage ($2000 2.5 years ago)
-Seats & harness (prices vary)
-Skidplates (not req'd, metal engine guard, plastic under floor guard, $250)
-Intercom (not req'd, but highly recommended)
-Rally tires (see www.taborrallyteam.com
for some current pricing and sizing)
-Mud flaps ($60, I buy urethane sheeting and cut to fit)
-tow hooks (have the cage builder do these)
-fire extinguisher, spill kit, first aid kit.
Honestly, that's all you need to do to rally a car. However, what other people have suggested will make you car lighter and stronger. Seam welding is pricey, so I only did the areas near to the suspension towers. Lights are almost a must have, but now days, some rallies hardly run at night anyway. I would recommend making a custom light bar, just make sure your mounting system is well thought out, so you don't get bad vibrations, or a crushed radiator if you hit something. One thing about the cage: if you can tie into the front strut towers, this will make the car much stronger. But it will cost you extra $$$ and many people don't do it and their car holds up just fine.
One thing about Coilovers in the rear: clearance is very tight. I would make sure to get a lower offset wheel. I've been using the Al. alloy 15x6" rims w/ 52.5 offset, and they just barely rub the spring. So, ideally, you'd want wheels with less offset, or spacers.
Drop me line if you have any detailed questions.
Brakes: The ford racing rally brake kit, I believe is just the disc brakes and hubs/knuckles off an 05 zx3. When I had to replace my pads, this car seemed to meet all the fitment requirements.