I may not be able to answer your question fully.
I do have first hand experience of a similar matter.
I have a 5 years old Dell Inspiron 1420 pre-installed with Vista. Less than 6 months ago, I bought a windows 8 Pro upgrade software from eBay, installed it then later upgraded to Win 8.1 Pro.
Installation went smoothly without issues.
Now, the Staples person is correct, more or less. Upgrading from Win 7 to Win 8.1will be a breeze.
However, in your case, THE BEST way for you is to upgrade to Win 8 first, then use Windows Store to upgrade to Win 8.1 which will be free of charge.
Why ?
Because : Windows 8.1 isn't designed for installation on PCs running Windows Vista or Windows XP.
First, you need to run the Windows 8 upgrade assistant tool to make sure your computer is ready/compatible to run Windows 8.
excerpt :
if you're running Windows Vista or Windows XP, you can use
Windows 8 Upgrade Assistant to see if your programs and devices are compatible with Windows 8. Most programs and devices that work in Windows 8 will also work in Windows 8.1.
Second, once you get the green light , shop for a Win 8 or Win 8 Pro upgrade software. Install and activate it. Then go to Windows Store for a free upgrade to Win 8.1
The official Microsoft site for win 8 is as follow:
Windows 8 ........
click this link Windows 8 Pro ....
click this link Note : I gave you the Microsoft links because some people frown on buying software from ebay.