The AP kit is 368mm on the front. I don't know of any other options in terms of rotor size. With the rotor, you have a choice of slotted-only, or slotted and drilled. Usually there's no cost difference between the two, but if you're going to track the car regularly, people say slotted-only is better because the rotor is stronger/better than the drilled rotor for tracking. As far as aggressive looks, many people like the drilled looked.
As far as the strap drive system, I can tell you it is not delicate or fragile. I've done almost 100 pad swaps in the last year on the fronts, and I've never had so much as a hiccup from the rotors. I know AP puts the fear of God in you with that documentation, but I've actually changed the rotor rings myself. That strap drive design is a piece of artwork.
I have never tried Carbotech pads myself. I would love to try a bunch of brands, but it's expensive, and you don't burn through them super fast, so you can end up with a set for awhile. I guess you could sell ones you don't like, but the AP kit is kind of rare, so there's not a ton of guys out there buying used pads.
I can't comment on the size of the Brembo pads, but I can assure you, stopping power with the AP kit is NOT a problem. I have funny story about having an instructor in the car with me. When he was in the car with me, I had already had the AP kit for awhile and had built up the trust in them to the point that I use the hell out of them. I late-brake fairly aggressively because I know they'll be there for me. (Whether the tires will be there is another story.....
) Plus, I have a blower, so the car's going even faster than stock would. So, we're at Willow Springs which has a fairly long front straight, and we do several laps---all in complete control, not a single out-of-control moment. Now this instructor is an INSTRUCTOR. He's not a guy looking for free track days. He instructs for teams, etc. So, he gets out of the car, and we're talking and he's giving me notes. And then he stops, looks at me and smiles and says "you know that you're using every ounce of those brakes right?" I laughed and said "yeah I know. I trust them. They have never failed me in any way, so I really work them." He smiled again and said, "yeah, well I tell ya, I've been around the block, and you gave me a couple sphincter moments there....." But he didn't tell me to back off. He was just confirming that I was getting everything you could get from brakes out there. And again, I will repeat that different pads give different stopping power, so if you want more stopping power, you can switch up to a more aggressive pad.
The pads for the AP kit have 12mm of pad material depth on them. I have found, and others seem to as well, that doing HPDE days, they are good for around 10 track days.
Couple other pieces of advice:
-There have been threads on this forum about how cleaning solutions can ruin any BBK brand's paint job. I have made the decision to NEVER use a harsh wheel cleaner. The fact is the brakes are subjected to quick and wild temperature extremes. The paint jobs have to endure this, and I think it's a tough job. Go easy on the paint jobs with cleaning chemicals. I've had mine for just over a year now, and I just use the car wash soap, and the paint has been holding up well.
-You need to come up with a reliable way to remove wheels so that when you pull them off, the barrels don't come crashing down on the calipers and scratch them. Sadly, you can't control every wheel removal and when other people take them off, they still get scratched.....ah well......
Now all you have to do is choose a caliper color!!