Originally Posted by
WIRunner
Just as a heads up though, if you buy a new machine with W10 installed, that does not have downgrade rights available, you will access to a W7 licence. Many of the machines that have been suggested have W8 or W10 on them, and a roll back may not be quite possible. You may want to narrow your search (or possibly expand it) to include machines with downgrade rights.
In general, most new business line machines with Windows 10 Professional will have downgrade rights. In practice, even if sold with Windows 10 (non-Pro, what used to be "Home") most Dell and Lenovo machines will have a royalty-OEM key in the BIOS, and be downgradeable in practice even if not technically licensed for it.
Keep in mind that on the next generation of machines post-Sky Lake (coming out late this year, probably early next for real retail availability) will officially be unsupported with any earlier OS, and while it's not clear that means you can't use Windows 7, it does mean that driver support is going to be a lot more limited.