the interior was redesigned for 2006. I believe 2006+ navigation is standard, too. I am pretty sure the drivetrain and body are the same from 2005 to 2006 until I think 2008 or 2009. I haven't really researched the newer ones.
If you get a 2003-2005, navi is an option and there are several different packages. Tech package has navi, intelligent cruise, backup camera. Sport package has 20" wheels.
My 2005 seems to have really good leather. I think a lot of it might depend on the past owners -- their size, eating/drinking in the car, if it's garaged, how often they clean or care for it.
Here is the 2005 Warranty info according to Motor Trend...
Basic 4 year / 60,000 miles
Drivetrain 6 year / 70,000 miles
Roadside 4 year / Unlimited miles
Rust 7 year / Unlimited miles
I also got a warranty letter today about the dash. Apparently, Infiniti decided they heard enough complaints and made the dash an 8 year unlimited mile warranty.
I would say you should probably look at buying something a little newer than mine.
I got a really good deal on my 2005 (owned it for 2 years) with low miles and the color I wanted, otherwise, I would have picked up an 06. I looked for several months, but there is only one Infiniti dealership in the entire state of Oregon and maybe 2 in Washington, so there aren't many FXs around. I had to drive over 100 miles to get it. I am very happy with it and not really mad about the navi. It would have cost probably $1k extra and I bought a portable one for $100 that I use for rare trips to places I feel I might get lost, plus I can throw it in my old Maxima or truck if needed.