I can't speak to the A/C. The user "Rancho VX" has a great gaslog saying what entries used how much A/C and it's impact on FE. Search for him and you'll find it.
As for the "buying down the miles" idea, consider this:
http://www.worldcarfans.com/10606206...th-anniversary
He's getting near 3 million miles. No major overhauls. No body rot. (he stores it outside a few blocks from the ocean)
Read up on this guy and his volvo. Now this is not exactly apples to apples, but it certainly expanded my thoughts about what is possible in a car. Miles really don't matter if you keep up on the maintenance and keep aware of when a rebuild might need to happen. These cars are sensitive and require lots of little things to get and keep them to a good FE level. I'd keep the old one and buy the new one too. Stockpile for when the price of fuel goes up. Sell it then (or part it out for your own car) and you win both ways.
B