Here's our experience, for what it's worth:
We have a '74 Overlander with 40+ year old gaucho that looks a lot like yours. I've done some sewing in my day, so we, too, were thinking of getting some new foam, making a pattern from the original covering, and doing the reupholstering ourselves.
We found an online company from which to buy upholstery remnants at a fraction of the price. It's called Toto. They'll even send free fabric samples. http://www.totofabrics.com
We were able to purchase remnants of $89/yd upholstery for a small fraction of the retail price. They sell small remnants, especially, at a real bargain. We bought several remnants (8-10 yds ea), so we could match the fabric's patterns. We also wanted enough for the bias-cording on the edges (which our original gaucho had). And, of course, we wanted extra in case we made a mistake. :-)
However, the more we studied the project, the more we felt like we might be wiser to have a professional custom upholsterer do it for us.
We took the whole gaucho (with the old fabric still on the cushions) to an experienced upholsterer. He told us later that even he found it a bit of a challenge.
By the way, we also asked the upholsterer to order replacement foam, as our old foam was beginning to break down. He says this new foam should last for decades, even in harsh environments.
Apparently, there are a lot of different types and grades of foam out there, so we were glad to have his expertise on that. He also covered all the foam with a layer of Dacron, to help prevent abrasion between the foam and the fabric.
We were glad for those little touches that we might not otherwise have known about.
But it was expensive. The cost of the whole project, fabric, foam and custom upholstery, was just over $900. That's a lot, for us -- a LOT. In fact, it's probably one of the biggest purchases we've made, in our restoration project.
But it was important to us to have an attractive, durable couch. We wanted a fabric that can take wear and tear and that is also stain resistant and easy to clean, and we wanted foam that is a comfortable firmness for sitting, as well as for sleeping. Hopefully, the couch will look good, and hold up well, for decades, like the original did.
I hope there is something here that's helpful. Whichever way you decide to go, I hope your project turns out great! Good luck with it!