I'm not a re upholsterer but I take on projects like this so I will just tell you my thoughts.
The couch if it is a stand alone couch will be a big job. Personally, not one I would take on. If the couch is just a bench type with a back and lower cushion then I would be more inclined..
The key to reupholstering is to make sure the sewing of the cushions is the same as the original. When the original is removed separate it carefully and make a pattern from the original pieces.
A measurement of the benches now should give you are good estimate of the amount of material. When the material is off you can get an exact measurement. You must allow for the matching of any pattern in the material. Side pieces that need to match the main top piece will take more material in a pattern to achieve.
Another consideration is the condition of the under lying support material, cushioning, webbing etc. If that is bad or too warn then you need to replace it as well, or as in the case of the people who reupholstered my truck they added some material (foam) to rebuild the seat.
I see the sewing of the material in the correct pattern as the most difficult part of this job.
Random thoughts might help you think it through. Good luck on your project.
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