What the question said also it'll be Windows 8.1 because that's the only one you can download from the M-soft store now.
The answer is simple and has been stated many times before. Boot Camp will not allow you to install to an external drive. Until now, Windows made it nearly impossible to install on an external drive, although I hear that has changed with Windows 8.1 (but I do not have personal experience with that).
If you use a different EFI boot loader, you might have much better luck installing Windows on an external drive, but Boot Camp does not support that.