Macbook Pro Retina external HDD
You can't run Bootcamp from an external hard drive, but you can install Windows via a virtual machine (MV Ware or Parallels are the best known). If you install Windows on the internal drive you have the option of booting from it directly or using one of the VM machines while running Mac OS.

As for the hard drive size you should consider the 768GB drive if possible. The drive space goes quickly and isn't upgradable. (Well, it is, but AFAIK nobody makes larger or faster replacement drives.)

The OSs run independently of each other, so you don't have to worry about cross-contamination. When running BootCamp you can't even see the Macintosh partition.

In my experience it works as well as can be expected considering the limitations of running software on hardware that wasn't specifically designed for it.

Good Luck.