My observation is that the Windows 7 version of the same PC is usually more expensive than the W8.1 version. We bought a HP desktop nicely equipped with i3, 6GB of RAM and a 1TB hdd. It was $299 with W8.1 and $499 with W7. We added a 120GB SSD for $69 to the W8.1 model and that system really flies.
I would also recommend to go with W8.1. It is faster and has some nifty functions you do not find in W7 - e.g. Refresh. I personally don't like classic shell because I never liked the old start menu. I think the start and app pages are a lot better. But I still use Control Panel rather than the modern settings UI.