Buying an '08 Q7 - have a few questions

I’m going to be buying a 2008 S-line Q7 tomorrow with the 4.2 FSI V8 and have a few questions for anyone that knows the answers:

  1. It has the 21" S-line wheels on it and they are fucking trashed. It rides pretty harshly for what I would expect from an Audi. I’m guessing most of it is due to big wheels and thin tires, but I’m sure the suspension plays a part also. If I buy new wheels, what it a good middle ground with regards to ride comfort, tire price, and looks? I’m thinking some 18"x8" BBS SR wheels with 255/55-18 touring tires. Any opinions?

  2. I have done a bit of research and haven’t really found anything bad about the Q7 variant of the 4.2 FSI. I’m assuming it has the same carbon problems as the RS4 version, but haven’t seen much info on it. Any other long term problems as these engines age?

  3. Any helpful hints on improving gas mileage? I’m going to be doing the normal stuff like plugs, low resistance tires, etc but was wondering if there is anything off the wall for these things. I’m not expecting it to match my A4, but a solid 20 mpg or better on the highway would be great.

  4. Is there a difference between the suspension with S-line and base suspensions? I do not have the air suspension but the suspension stuff I’m seeing says that it fits Q7s w/o S-line and w/o air suspension.