Even for doing the CG version of traditional art, I'd recommend to try Krita or MyPaint : They propose individual tools "Watercolor", "Large ink pen" etc. which is much more handy than having to customize Photoshop's single "brush" to reach the same behavior.
Actually this is false: For both Krita and MyPaint, brushes near-entirely determine the painting behaviour. The only 'special tools' are precision-related: eg. drawing straight lines, connected straight lines, curves, ellipses or boxes; they affect only the (virtual) pressure and location that the brush is applied with.
Photoshop (or GIMP) actually has *more* individual tools for different paint types than Krita and Mypaint : Pen/Ink, Smudge, etc.. which are all just part of 'the brush system's of Krita and MyPaint. Personally this is why I find Krita and especially MyPaint's painting more approachable and understandable than other apps.
LarkoftheRiver:
It should be noted that it's also the case that a number of the programs proposed here are free or have a trial version :
Free: MyPaint, Krita, GIMP, GrafX2, MTPaint, Aseprite
Trial: GraphicsGale
It may be getting slightly OT but
Blender could also be put on the Free list -- I've been finding recently it's great for constructing refs of 3d forms or even text -- with the large arrays of addons and non-destructive modifiers that can be applied to an object, I think of it as a kind of 3d inkscape in terms of it's capability for constructing clean/consistent mockups; With BLAM, it's possible to directly match the camera setup to the camera of your drawing. Also
Inkscape itself, which is more on the drawing side than painting.
While I'm being excessively complete -- the GIMP-GAP suite of plugins for GIMP I have also found extremely handy for constructing animations. It's video-oriented, so you have to keep in mind that you don't 'set frame time' -- you just duplicate frames if you want them to show longer, and the actual time frames are shown for is determined by the FPS. Pretty suitable for longer or more ambitious animations (eg, if you want a scrolling background with objects animating in front of it..)