I'm trying to decide what Windows/parallels set up to go with on my new Mac. The only thing I will use the windows system for is to run Quickbooks pro and possibly Microsoft Office.

When Windows is purchased does it come with a disc that has to be ran to install? Or do they come with a product key code? Obviously my Mac doesn't have a disc drive and I do not have a usb disc drive.
I see that some versions of windows 10 can be purchased with the program coming on a USB drive.
I thought I would purchase windows 8.1 since it was cheaper and can be upgraded to windows 10 for free. But unless i'm misunderstanding, windows 8.1 needs a disc drive to be installed.

Next question is if Parallels 10 will run windows 10. It is about $37 cheaper than parallels 11 right now.  
I know I can download parallels 11 and then order the windows 10 with usb. Just thought I could save myself a few dollars by getting older versions and put those dollars in Santa's piggy bank for the kids.

Its too bad acer won't let you take windows from there computers. I have windows 8.1 on a small touchscreen junk acer. But in my research I've seen the product key code is locked inside the computer and can't be gotten out.