We ended up buying a PaceMaster Bronze treadmill from someone on Craigslist. PaceMaster is only sold in fitness stores and it was bought 3 years ago for $1600. It is still very solid and smooth. Hardly any bells and whistles, but my dad doesn't need them. No warranty though because warranties don't transfer. We got it for $430 shipped.
Before buying we went to some stores and looked at stuff in the equivalent price range($400-$500). All of them were significantly less sturdy, had weaker motors, very plasticy, and I had the feeling we were paying for too many bells and whistles. There was a $500 New Balance 2.5 CHP deal on FW but it seemed a bit risky... reliability seemed a bit low and repairs took months from some reports.