I have a 21 mile round trip on roads and bike paths (asphalt). I sometimes hit the trails for an extra 5 miles if I am to meet folks from work at the pubs.
IMO, a mountain bike would get old for me for 21 miles a day.
I bought a steel cyclocross bike. It's kind of a swiss army knife. Good commuter over any surface with the proper tires. And to boot it is used as my Sept-Dec cyclocross racer.
Getting a steel CX frame may be a good start - I'm sure there are deals to be found, especially now as CX season just ended.
As far as parts go, a solid wheelset and drivetrain goes a long way to minimize the repairs.