I don't think too much fancy brushes are used, most of what I see can be done with the standard Photoshop brushes. If you want to know how te retouch photos (I think a lot of people prefer a bit of a subtler approach than the puppet/robo look of M.E.), there are plenty of tutorials to find online. Search for "photo retouch tutorial" on Google and look around. No need to buy brushes. You might want a Wacom tablet.