Newsnake,
Here's a photo of a car designed from the beginning for an undercar exhaust:
Note the headers designed for this application and the notch in the outrigger support to allow the pipe to be moved upward an inch or so.
I've driven this car 27K miles with this system and have had to rebuild it twice, completely, including the headers which were poorly made at first. The latest system looks very much the same - the workmanship is far better - and has lasted more than 15K miles just fine.
Here's a shot of the whole car:
The exhaust is not the lowest part of the car. The bellhousing and deep automatic trans pan share that honor.
Unless you have good other reasons, try to find a car or kit that is set up for this type of exhaust from the beginning, particularly if you intend to put some significant miles on it. If it's just going to be a garage or trailer queen, almost any setup would be OK.
There's no chance for "snakebite" with this setup. The noise level is still, um, significant. If you can run the exhaust out to the rear of the car, it will be quieter. Alas, on this car there wasn't enough space to get by the Jag rear.
So you pays your money and takes your choice!
Enjoy,
Tom