My middle garage door panels are damaged. Do I have to buy a whole new garage door?

No, as long as the garage door panels are still available, you can have only the affected panels replaced. You should consider that the cost for parts and labor to replace the affected sections could be close to the cost of having an entire new garage door installed. A new garage door installation will usually come complete with a new garage door, garage door springs, tracks, rollers, cables & drums.