It would be best if you contacted a couple of your local furniture upholsterers for the correct answers to those questions because it's going to depend on the type of sofa and the type of springs it has.
I would ask the upholstery shop if they have suggestions. Even if you buy fabric at a discount store, a specialty store might help you. I know that sometimes they aren't needed. My mother recovered my father's Packard seats by retying, as she did with a very aged sofa. Most of the time, they aren't ruined, just pulled apart. Heavy grade twine between each coil and then sort of webbing will often salvage the old ones. (07/13/2009)