garage door opening mechanism

Hi All, This is my first post here ever, so please be gentle with me! I have a double-garage with an automatic "up and over door" which is operated by a key fob: I reverse my car into the drive, press the button, the garage door opens, and I reverse into the garage, then close the garage door...

One of the springs or cables on my up and over garage door has snapped off due to having been heavily corroded. Which tradesman would be most suitable for this type of work and as an estimate how much would it cost to have it replaced?

I need to replace my garage door. I have the chance to buy a garage door cheap 14ft wide however my width available is only 13ft and can't be easily changed. Is it possible to have the garage door so I can mount it behind the opening and the door will open up and over without protruding...