OP:
- Use an online gear calculator to determine what gearing you need, after riding any bike that has the low and high gears you desire.
- Make sure the new crank arms are the same length as before, unless you intentionally want to change this for your leg length. The trend in recent years has been toward a bit shorter crank arms.
- Make sure the "Q-factor" is acceptable for you; This is lateral pedal spacing, and mountain cranks typically put the pedals farther apart than a road crank, in order for the crank arms to clear the chainstays.
- Make sure the chainline is correct with the new crank. This is distance from center of frame to center of chainring(s), which should line up with the center of your cassette. For example, my road crank on 7-speed 130mm O.L.D. rear, has 43.5mm chainline.
- Look at your options; When I upgraded from a single crank to double, I tried the 2-piece hollow-spindle design with external bearings, and LOVE IT, for many reasons I find the setup superior over taper spindle bottom brackets. For the external bearings, try to choose a common standard, as each design requires a different wrench; I went with "ISO External" standard, very common.
- Make sure that replacement chainrings are low cost and available for the BCD pattern of your new crank.