First of all, physical appearance of the machine is a good indicator as to whether the machine has been properly cared for, but not always a good indicator to whether or not it will be a good machine. Anything can happen to any machine, new or used, at any time. Granted, the chances of having problems with a new machine are far more slim than a used machine.
I'm going in circles....About the "hours" on the machine. Are you talking about "power on hours"? Are you talking about "spindle hours"? Or are you talking about "in motion hours"? The machine can have 10,000 "power on hours" and not have many spindle or in motion hours. A single shift shop can add up 10,000 power on hours in 5 years, assuming 2000 hours per year. Our Mini Mill is almost 6 years old, has over 850,000 tool changes, and the last time I looked, over 14000 power on hours, and still runs with the best of them. But it has never been crashed. It has run wide open at 6000 rpm for more hours than I'd like to admit. But I try to keep the load meter to 50% or less for anything we run. Of course, try getting a 50% load on the spindle with a 1/8" end mill in stainless. Not going to happen.
As to a life span... I never really asked a Haas rep what they consider the normal life span to be. A rep from Hardinge told me that they consider the lifespan of their machines to be: 1 shift, 10 hours a day, 5 days a week, 52 weeks a year, for 5 years.
We have bought both new and used equipment and have had good and bad experiences with both. When it all comes down to it, only you can make the final "yeah" or "neigh" on the machine. Like buying a used car... Would you be afraid of buying one with 100,000 miles on it? 150K? 200K? I'd take the one with 200K over the one with 100K if the one with 200K had been taken care of and the one with 100K had been beat on.
Look at it.
Listen to it.
Have someone else look at it.
Have someone else listen to it.
Have a ball bar test done on it if it makes you feel better.
If it's running in a shop currently, look at a sample of what it's making.
We have no idea how many hours our Citizen swiss machine has on it because the batteries in the control died and it lost it's memory before we bought it. In the last year and a half, we replaced one servo drive with a NOS unit off ebay and had one rebuilt by the manufacturer. Mind you, the machine is 20 years old. And we paid less than 10% of new price for it. Still, it's one of the best machines in the shop.
Good luck in your hunt!