If you have the serial number of the machine, I think you can phone Haas and find out the repair history (if any) on that serial. This may give you some indication of its past.
400 hours is nothing. I don't think anything will be broken or worn yet. But you can always look it over for obvious oil leaks, and overall cleanliness.
Did the previous owner mention anything about warranty issues he'd had?
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)