Link to Photoshop, or any app that can open PSD files
You can save and open your texture work in apps like Photoshop, work on them and send them directly back to 3D coat. It will remember your layers and update.
Professionals recommend 3D Coat for Handpainted textures
You can texture in many styles with this app, but when it comes to Handpainting, as seen in World of Warcraft and Torchlight, 3D Coat is the one that many professionals will advice you to as their top choice.
While sculpting in polygons you can have 3D-Coat adaptively tesselate the mesh for you, based on your input, or turn it off entirely. All as you require.