Brushes, are like wax, there is no one size fits all. Different brushes are used to achieve different results and what results you wish to achieve will be dependent on the conditions you will be skiing in. Hard wax, soft wax, overlays, powders, then you have the type of snow you're going to be on. Is it wet, dry, hard or icy. Is it new snow or old snow. Is the air temp warm but the snow is still firm and cold. This would apply for racers only but has the course been salted. There are sooooooooo many variables.
What I'm getting at is even though we are only talking about brushes and you would think this would be a very simple a b c answer, it is not quite as simple as you would think and the more you know about conditions, waxing, brushing, and tuning skis, the more complex the answer/s get.
I'm sure all the tuners on this forum can remember back to when they first started working on skis and had 1 wax and 1 brush. Things were so simple then, they certainly were a lot cheaper. But, then we screwed it up and started learning more about waxing/brushing for different conditions and we continue to learn and the wax companies continue to come up with new and better products so it's pretty much always evolving.
By now, the OP is probably thinking wtf, can't someone just say use these 2 or 3 brushes in this order and be done. For the most part, yes, we can and Jacques suggestion of steel, brass/copper/bronze, horsehair and in that order will likely achieve what you want for a wide array of conditions. Remember, always let the brushes bristles do the work. Applying light to medium pressure, depending on the type of brush, is ok but pressing down hard is not what you want to do which is why I always tell people, brushing is just like filing, let the tool you're using do the work not your muscles.