Regardless of whether you make your own or buy your brushes, proper care and storage is important to keeping them in good shape. Many folks store their brushes handle down in a cup/jar, with bristles in the air. That is not a good way to store or dry your brush as the water runs down into the ferrule. Store your brushes by hanging them from the tip of the handle. That lets the water run down when drying and it allows the bristles to keep their shape when dry and not in use.
As mentioned before, use cheep stuff for wax (personally never used wax) and good stuff for painting lines etc.
Natural hair (dog, squirrel, sable, ox ear, pony etc) watercolour brushes are the best. For a really fine lines, I actually use really thick (>10) but fine tipped brush.
Forget the synthetic bristles. Yes, some of those will soak up paint/gaze well but will not let it go so well as natural hair does.
I got myself few Escoda pony hair round oval shape brushes with short handle. Wow... I wish I had the money to buy more of those
They will probably last for a long time.
To be honest, covering large areas with brush is, simply put, BS and I like to avoid it if possible. Pouring, dipping, throwing o_O or what ever you can come up with, will give better results. I wish I had a spray booth
I got this idea from Marks pictures of his very nice, minimally bloated mugs that he tossed in the trash. I was thinking that mine would have been used as tool or pencil holders, or whatever. He's been doing this so long he is overflowing no doubt with vessels used to hold just about anything.