Not sure that I quite understand the last part of the question. If you buy a new computer with Win8 installed yes you can delete it and install Windows 7 instead but of course you would have to buy a copy of Win 7 to do so. That being the case you could buy a computer with no operating system installed and install Windows 7 yourself.
What are you using to back up to your external hard drive? Windows own built in back-up or another proprietary software such as Acronis or maybe are you just dragging your User files to the external drive?