This is an automatic steel garage door that is sectional (the most common kind)
What would I use to seal the seams?
Would MDF be good and cheap soundprooding?
If building a second door instead is cheaper is there a way to make this practical to open easily?
I am willing to rebalance the door and maybe get a new spring as it pretty much needs one anyway.
Your best bet is to buy sheets of styrofoam which they sell at hardware stores as insulation. You can cut the sheets relatively easy and fit them in the back of the door. Use duct tape to keep them in. Do not use mdf or any other type of wood as they are heavy and will effect the operation of the door as they change the weight and the spring on the door will no longer be effective.