It would be a rare find indeed to find a new computer with Win 7 installed.
Yes you can install Win 7 fresh on a system that came with Win 10. There may be issue with perhaps drivers for hardware that is so new it only has Win 10 drivers, but I don't think we are going to see much of this just yet with average computers.
The key to success is you will have to completely wipe the system before you install Win 7.
As for it trying to upgrade to Win 10, since the free upgrade time period has passed, I do not think you will find it trying to upgrade automatically to Win 10. It may from time to time still try and suggest you buy a copy. I can not say from experience as my Win 7s machines were never upgraded, and they neither ask me to update as I never allowed the update to Win 10 code to be installed on them. My Win 10 machines came with Win 10 already installed.
It may be cheaper and much easier to deal with a system that already came with Win 7 even if it is refurbished.
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