I own a '86 300E and have asked the same questions about restoring the exhaust manifold. What I ended up doing is buying white high temp spray paint. The result was good and the paint is very inexpensive, a quick and easy solution. I did not remove the manifold, I carefully masked things off and it looks decent. Another option is jet hot but I don't think you can get it in white. One suggestion was to get the manifold ceramic coated in white but it is not a cheap option.