The WF plastic zombies are okay for a horde. If you are looking for more unique undead, other options are probably better. Vampifan's blog might be the most comprehensive collection of zombie miniature reviews.
The HorrorClix line has quite some models that would fit right into RE, one is a purple humanoid with no skin, another one is the Rottweiler, a pink mutant dog-like creature with spines on it's back. Hasslefree has the Jotun which would make a fine Nemesis mutant.