The first thing you never want to do is sand blast the roof, it tends to leak with all the new holes.
Sanding is an option,just be careful up there when you do it or you will slide right off with all that fresh dust.
Try a company that refinishes roofs and see what suggestions you may get from the"experts."
If its to much woork or the roof is in worse shape than you describe, reroof.
Some folks think those rusty roofs add charm,