Re:
Aluminium alloy. Generally good and strong. Not many of these has cracking problems. Such is aluminium, they can feel a bit stiff to ride, but comfort can be reaped back with front suspension.
Also, the frame can be built up with common components. Such as 68 mm bottom brackets, 1.1/8" forks......etc...
You won't have to spend a million bucks either. Shimano LX, STX etc. will suffice.