Having just done a whole house which pretty much similar surfaces I came to
the conclusion if the walls are really bad then geta pro to skim it for you
fro the other walls, patch it up sand it paint it then you will see more
bits that need patching areas that catch the light so then fill and sand and
paint again, takes a while an may be a botch job but does come up looking
quite good. Oh and a good quality paint for new plaster worked quite well