I paid less than $9K for mine with 99K mi. Don't go over 10, nav adds $? big woop, play it down. Tell them you're more than happy to get one without nav and buy a garmin for $200.
25k/year is a bit high, but I'm guessing that's mostly hwy? That's what these cars are good at.
So far, I've put another 11k on mine and the only real problem I had was the classic cd player that drains the battery problem which I got around for free.
Most importantly, if you decide to get a G35 ALWAYS look here first if/when you do run into a problem, someone on this forum has likely had the same thing to deal with, and it could save you hundreds if not thousands.

 

I drive about 25K miles per year as well. I bought a G with 26,000 miles on it because I wanted a.) to make sure the maintainence was done on it throughout it's life and b.) because i could get a few years out of it before needing to start worrying about replacing shocks and struts and doing major maintainence.