Mike,
My initial thoughts are the deals you received are not unbelievable, but also not out of line either, based on my quick calculations. With regard to the two machines, the 485 is a good bit of an upgrade. While both are quite nice and share frame, etc., the fuel injected engine is more powerful, easier to start, a little quieter, and somewhat better on fuel. I think it would be worth the extra money to upgrade. With regard to how I liked mine, I did. I traded it on a 595 with a 62" deck, which I liked better, mainly because it was diesel and had 4wd. The X485 would have been a good machine for me for a long time had my property needs not changed. Even with RWD, it is a stout machine that has definite ground engagement capability, and mows nicely. I think you would be happy. One note: Kawasaki engines are noted for their power and engineering/reliability. One thing they are NOT noted for is their fuel efficiency. Having operated several of them, as a group they seem to get the worst fuel economy of any small V twin garden design. The fuel injection will offer a solid improvement in this area, but just know these particular engines are thirsty, even the fuelie. Good luck.
John M