Sounds steep for that number of documented hours.
How do you know it's at least 400 but not more than 500 hours, may be nitpicking, but if the man 'knows' the number of hours, why can't he get a closer actual?
Is there service documentation to be seen?
Extra blades and sulky are great to have, but I wouldn't pay more for a mower cause it had them. I wouldn't care care how many bald employees came with it.