Angels have the Phone Box
Can't say. I would be leery buying from sites like that - you never know if someone got hold of a leaked key, which will eventually be blacklisted by MS in the future some time.
You could just go ahead and update, after making a Macrium image of your W7; or, just install the new SSD, clean install W10 and use the W7 key to activate. You have 30 days to revert. Personally, I'd take the free upgrade now, rather than spending $200.
I just upgraded a W7Pro x64 system this evening. Took all of 1 hour and 15 minutes (after prep), and it's fine. I downloaded the ISO from TechBench, installed 7-zip, used it to extract the ISO to the desktop, ran setup.exe, and off it went. Couldn't be easier.