TC
I went with German engineering when I built my double garage 8 yr.'s ago. Hormann (pronounced 'whore man') insulated double door with electric motor. I think the motor added £300-400 to the overall cost, but its been one of my better ideas, use it every day - never failed to open (expect when we had a power cut - Yes we had payed the bill)
Dont know if you can retro fit to your existing door, think it depends very much on the door and (don't know how to say this) how much headroom 
you have
http://www.hormann.co.uk/
Thats what I've got as well attached to a sectional insulated door. Kevlar maintenance free drive belt and you can have a few switches wherever you want for opening it. Once its closed a metal rod drops into the mechanism which would make breaking in through that door nigh on impossible. Seems a good bit of kit so far.
Don't loose the remotes though as they are hellish expensive to replace.
