you ate the cookies didn't you
the beauty of the new dex is that it focuses on renegade chapters as much if not more than the traitor legions, which means that all you have to do is repaint you marines and they'll fit right in. for a really simple finish just paint over the imperial imagry with chaos star type dealios and bam, instant renegades! converting works well too
for doing a nurgly theme you could just go for a dirty paint job, of you could check
this out. the bit with the sand is so damn easy and should add just the right touch of nurgly goodness.
as for the paints i never use anything but citadel colour (gw brand), i swear by em. yes they are kinda expensive but they are good quality and easy to use.