I'm sure there are better ones out there, but if you are looking for "bang-for-the-buck" they have a 1/2 HP Stanley at Home Despot for $99. I don't think the $99 one is strong enough for a heavy 2 car garage door though. They have very good instructions and also a phone in help line where you can actually talk to a human with any problems. I have a couple and they have been fine.
This can be done by someone who is handy. The toughest part would be putting the carriage bolts through the door and also the hanging of the motor unit with the metal braces. I think the most important thing in having a safe install is the condition and adjustment of the existing door. The door needs to have completely free movement with no jarring, jerky ups and downs. The tracks need to be in good shape and the wheels should be lubricated and should travel freely and smoothly. Also, the spring tensions probably need to be adjusted so that the door holds itself up if brought half way up and you let go. How the door and springs should be balanced is detailed in the instructions (not how to do it but how it should end up). CAUTION!!!!: It is possible to maim or kill yourself if you don't know what you are doing with garage door springs. Don't adjust them yourself if you only "sort of" know how to do it because fingers, hands and eyes etc. don't regenerate themselves if lost.