>>324252819
1. You'll need to configure the materials/shaders in unity. Most textures that come with models will work fine with the common shaders though.
2. whatever 2d image editor you prefer will work fine, and blender texture paint mode can be good enough for simple edits. I think most people use substance painter for 'real' edits, but it's expensive so i've never bothered.
3. I don't know a specific guide, but I usually just use a lot of proportional editing if it's loose clothing, or shrinkwrap if it's form-fitting, then transfer the weights.