You don't need the Army Painter stuff for Skaven. Brown spray paint certainly speeds up the process, but spray and dip tends to leave sort of iffy looking models, I think. 90% of painting Skaven is learning to drybrush the fur, apply unusual colored washes to get stains and "filthy" effects, and learning how to do very basic layering on skin and armor.
Anyway, Skaven do well because they have extremely effective war machines. Weapons teams are mobile and do a disproportionate amount of damage for what you pay, and by the time you've chewed up your opponent with war machine fire, your blocks of crappy infantry (and make no mistakes, Clanrats -are- garbage... but garbage that comes in a huge mass.) can usually win on static combat alone.
Skaven also have access to a fairly cheap Level 4 wizard, and the spell list the Grey Seer has access to is quite solid.