Tablet isn't really necessary for skin texture painting unless you are doing design flourishes and drawing type tasks. Pressure sensitivity sure helps for airbrush and natural brush/pen strokes line weight effects. I'm sure there will be those that heartily disagree but I'm just commenting from my usage of both a mouse and stylus tablet combo.

Texturing forum is a ghost town last I checked.

Go with substance, it's more expensive than Mudbox but has more up to date tools, features, and development. Mudbox is still handy to have, for a variety of reasons. Not bad as an adjunct to substance, still one of the best traditional 3d painting interfaces IMO. Now that Substance is owned by Adobe it's moot mentioning Mudbox's direct bridge with PS. Not sure if Substance can do flatten to UVs painting like Mudbox... but

You're not using Photoshop but rather Gimp so I guess all of that's all moot also.

There's the whole advantage of PBR materials interface with Substance too. Frankly there's a heavier advantage there with architectural, environmental, and non organic things, IMO, as far as generating the maps for that type of rendering.

Tools are nice, practice is paramount.