It all depends, mines pretty low, but I put over half down and took some payments to build up some credit. As for the engine ticking, perhaps it's just luck of the draw, I've put 5000 hard miles on it l, and was doing 150 on the Auto Club Speedway with no issues.
Other than the standard GT brakes, they're terrible when you threshold brake coming off the straight away. Get yourself either the Performance Pack or the base GT and spend the 1100ish for the front brake upgrade, highly recommended if you track it hard.
ed: Crazy idea, purchase the extended warranty. I brought the car in with no brake pads and scorched rotors, and they fixed that and switched the brake lines out... no cost (other than the additional warranty, duh). If the engine is that bad, they'll do a recall, and if not, you can just keep complaining and lemon law it. Until then, enjoy a 460hp car that's faster than anything not named GT500, and the 11's if you know how to launch.