ultimas would be the best but aren't they around a $100k each w/o drivetrain? still worth it, just to kick enzo's ass but...
factory5 cars are well made. tough and robust. 7-8 year old issues aren't really issues. if you want a cobra REPLICA go look elsewhere and pay double. if you want a car that *looks* like a cobra, that's about all you get w/ factory5. they are primarily racecars built to a price point AND MADE TO ACCEPT mustang parts. it will never be a good replica of the original. the fact that there is a raging race class in nasa w/ these cars being run nationwide, should tell you about their reliability and cost of running. it is very much more developed than what's been created 7-8 yrs ago.
edit:
as far as stuffing 911 power into other cars, i'd love a flat-six-powered exige. you would get the grunt, the noise, a lower center of gravity (than stock), and having 6 stacks peeking out the back would be the bollocks
