I think it smart of you to be picky about what fabric you buy - getting good fabric means you will be happier with your projects, and they'll last longer.
Once you're more familiar with the brands that you like and are happy with the quality, you'll be confident to order from those lines online.
I wouldn't worry about building a stash - it will happen on its own! I always purchase more than the pattern calls for (grand ideas of making matching pillows or other goodies, allowing for miscutting and pattern errors).
But I certainly have lots of purchases where I just fell in love with the fabric and didn't have a particular pattern in mind - usually end up with five fabrics that go together, a couple of yards each, 3 yards for what might be a border fabric.