That's a tough one to answer because we all have different tastes and ways of doing things. I've been quilting forever and I don't remember how I got my large stash started!
I love fat quarter bundles. But you need some yardage to start off with. If I had it to do from scratch today I would buy some neutral yardage and my favorite fat quarter bundles. I love them for the variety and theme. But that's me.
You have been given some great ideas. Making kits? Yes! In fact I may do that with some of mine. I have a lot of patterns waiting to be made up. I like that idea.
I get tired of fabric easily so I purge every now and then. Send it off to a donation site. Lots of "what was I thinking" goes to them too.
These days I have most yardage I need and buy precuts.
I don't wash fabrics before using either, unless it's red or looks like a bleeder. I've had no problems at all.