Ok thanks. So basically this is what I am looking at doing. Getting a new MBP but with limited storage. OSX and Windows 10 through Bootcamp will be installed on the MBP, along with software that I use. Modo, Unity, Unreal Engine, zBrush, Substance Painter/Designer, Photoshop, etc.
However, the files that I will be working on with these programs will be saved and worked on in realtime through the external SSD. Some of these Substance files are in excess of 5 GB and the same goes with Unreal/Unity. They can > 20 GB.
Hopefully this will explain things properly, if I left anything out, just let me know! Thanks for your help!