Windstone recommends Golden fluid body acrylics, and I find them the best. They’re archival quality, thin enough not to obscure details on the sculpt, and have a depth/life that most acrylics seem to lack to me.
You don’t want to use heavy body or thick paints, as they can obscure details. You also don’t want to use oils, they don’t allow the gypsum to breathe.
I really don’t know the best brushes, sorry.
Using a retarder (Golden makes one) will slow the drying time of your paint, which helps. If you’re mixing your own colors, doing so in small sealable pots or jars may help, keeping them from drying so you don’t have to try and match color (as much).