| I've bought quite a few of the supplements, but I would be very happy running WFRP with the core book and Old World Bestiary alone.
I'm going to go off the reservation a little just to warn anyone reading this thread and considering WFRP that, if you are looking for a "rules lite" system, WFRP may not be the way to go. The combat rules aren't quite as complex at D&D 3.5, but they are still heavily tactical and require some level of rules mastery to run smoothly as a GM. The one area where WFRP is really strong is in its much lower power curve. This ties into what Elsenrail mentioned above, it's very easy to stat out NPCs (no matter what "level" they might be) quickly, which is a huge timesaver compared to D&D in my experience. But I still wouldn't call WFRP "rules lite". | 
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