2004 is an old machine regardless of the 160 hours. Still, I would guess between 3500 and 4500. Personally I wouldn't pay more than 3500... but that's just me.
I sold one of my oldest machines last year. It was a 2005 Z Master with about 1800 hours and had been used as a back up / reserve mower for the past several years. It also had the 23 Kohler but it was a 60" and they are easier to sell than a 52". Asked 3500... got 3200.