I don't know what the term Sayelle actually meant. I looked for in on the site called Vintage Yarn and found several. It looks like they could come in different weights. They were all 100% Orlon. I think some acrylics are called Orlon Acrylic. It sounds like it was just a man made fiber of some kind. You could probably use Red Heart if you want. But you wouldn't have to, you could even use wool if you wanted, you'd just have a nicer afghan, but need to wash more carefully.
Unless you know which weight they yarn was it is pretty hard to be sure, but I think most afghans are made of worsted weight. One way to make an educated guess would be to go by needle size if your pattern gives one. Vintage Yarn said worsted weight had a recommended needle size of 8, 9, or 10. According to Vintage Yarn, Botany made a worsted that had 140 yards yards in 2 oz. And Dawn made a worsted that had 258 yards in 4 oz.
Math is not my best subject, but I think based on 11--3 1/2 ounce skeins (which would be about 38.5 ounces) one of those would figure out to about 2401 yards and the other about 2800 yards. Could be that much I guess.
I think "Aran" is cream, or off white.