I think that CO drain is a little bit harsh, since it takes a very long time to heal, and the RR against it only protects very high level characters (who are actually immune to it), normal folks just dying in their beds when some ghost roams outside.
I copied Life Level system from one of those Companions (can't remember which one), converting it in mere Vitality stat that is always 100 when the living thing is healthy. Vitality is the value that drops when in close contact with undead. No RR, but many spells in Channelling Companion help. Any drop in Vitality causes penalties, ie. Vitality of 95 causes -5% penalties (in our game, it's a real percentage so every skill is at 0.95 x total bonus).
Vitality recovers automatically one point per day, or d10 points per day in the temple or holy ground. If Vitality ever drops to zero and stays there over night, the character wakes as some form of undead.
I guess Vitality could be used with bleeding wounds, thirst and hunger, but insofar I haven't done that.