Welcome!

The 3.8 is one of the best engines ever made. It will last forever with regular maintenance. Make it last longer by using synthetic oil, if you can. GM can't make a transmission to save their lives, so watch out for problems with that. You should be fine if you don't beat on it regularly.

You should net from 21+ MPG at the speeds you mentioned, but remember that the faster you're going, the less you're getting.

They didn't tone down the S/C engine for 04+. In fact, they ramped it up. It has an additional 20 HP, though the 280 torque stayed the same. They're just as much of a beast and it doesn't matter how many doors it has, it'll be blowing them off anyway.

People need to stop looking at the number of doors a car has; there are tons of sedans that can embarrass their coupe brethren.