I think fastlane Motorsport is an OEM Tuner, at least around North Carolina. But I know what you're saying and thanks for your advice. I'm going try and buy it tomorrow. Try and pick it up for around $29k otd. If not let I'll move on.
As much as I love to mod my cars, it's always nice when it comes from the factory exactly the way you want it. There is a 2009 Shelby only a few miles away going for the same price, 9k miles. But I'm afraid there's something wrong with it. No car fax/autocheck available on their website.
Even if Fastlane is an OEM tuner, consider this:
My '99 Saleen had a difficult time selling back in Spring 2008. The trade-in value at a dealer was no better than $2200, because it was 146,000 miles and they wouldn't recognize it as a Saleen; they would only value it as a Mustang GT, since they only use KBB, which doesn't list Saleen. I tried selling it privately, but buyers were all referencing KBB and not NADA Guides, so we were always far apart on valuation. Even still, NADA Guides doesn't take into consideration mileage, so it is not an accurate gage whatsoever as to the true value of the car. It took me months to find the right buyer. And this was with a widely known "name brand" OEM tuner. Imagine some of the problems you might run into with a lesser known tuner vehicle.
If this Fastlane GT is priced reasonably at the dealership per KBB's '12 Mustang GT valuation with respect to its mileage, then I would certainly go for it. If it has all or most of the mods you want done already and is backed by a dealer warranty, and if the price makes sense per KBB, it's a no-brainer.
As to two of your original questions: those mods should not pose reliability issues; $9900 for front and rear bumpers, side skirts, rims, axle-back exhaust, CAI, and custom tune sounds like $4500 worth of parts, so the rest is labor and dealer markup, which seems high to me, but if it is backed with a bumper-to-bumper warranty, that's a bonus you can't buy in the aftermarket.