The mistakes I have made with a stash- purchasing too much of something that was so-so, and not buying enough of my oh-my-gosh-I-can't-live-without-it fabric!
I agree with buying smaller amounts of the fabric that you "like", more of what you "love" and the whole bolt or as much as you can afford of that "can't-live-without-it" fabric. So rate each fabric by how much you like it before you deciding how much you need. When you have a pattern already in mind, and then it's how much extra you are purchasing past that pattern amount for your stash.
If I were starting a stash, then I would be routinely searching fabric sales, 2nds from Connecting Threads, etc, and plan to do that over a year's time- I would not plan to build it in one sitting, because you won't find everything that you like now.