The rear gear on these cars is 3.46.
Most any RWD V8 is going to be a gas hog. You can squeak a little more MPG out of a 6 speed. I've had friends get almost 30mpg on freeway trips. My '04 is an automatic with a larger cam and a stall converter and I get about 16 around town and 23/24 on the freeway.
As far as ride quality, it's been a long time since mine was stock but I remember it being a pretty smooth car - too smooth for a lot of folks. I changed out my stock shocks, struts and springs early on to stiffen up the ride. The stock exhaust is pretty mellow too. Again, I dumped my stock exhaust for headers and a cat back.
If the car is untouched, take a look at the front struts and radius rod bushings. Some of the early '06 cars had leaky struts from the factory. The stock fluid filled radius rod bushings are usually the first to go on the front end. Most folks replace them with aftermarket urethane pieces.
With 18" wheels you should double check the wheel alignment and tire wear, but you probably don't have much to worry about in the way of strut rub. That's typically more of an issue with the 17" wheels.
Check out the LS1gto.com forums. There's usually a number of cars for sale there, most are well cared for by enthusiasts.
http://www.ls1gto.com/forums/forumdisplay.php?f=20
I can't really speak to price since I haven't been in the market, but a friend of mine in the business told me used car prices on the whole are up.