Welcome to painting! Drybrushing can be tricky, but it can also be really flexible. By this I mean the kind of brush isn't necessarily as important and the state of the paint. I've drybrushed with anything from small cheap mini brushes to brushes used to paint walls. One thing you mentioned that caught my attention was the idea of thinning your paint out, and I'll tell you right off that this is a bad idea. When you're painting the mini proper, you'll thin you're paints with thinner or water, or what have you, but if the brush you're drybrushing with has even trace amounts of water, you're "drybrush" will bleed and blob and look sloppy. This is something that's screwed me up time and time again, and it can happen very often unconsciously. If you use a wet pallet, that paint is no good for drybrushing, because of the water at the base keeping it all moist. Just drop a spot on a piece of tile or something water-free and work from there. One reason drybrushing can destroy brushes is that you're intentionally letting the paint dry and clog in the bristles. Hope this helps, good luck!