There is a little thing known as Downgrade Rights. Basically what this means is that if you have Windows 10 Pro you can downgrade to Window 7 Pro without a great deal (comparatively) of hassle. But I have not see any laptops even remotely close to your budget which have Window 10 Pro installed on it. This means that you are going to need a copy of Windows 7 and a valid product code. If you don't have a new copy of Windows 7 and the valid product code you are going to be spending about $70 at a minimum to purchase it. Then you are going to have to wipe the hdd to remove all traces of Windows 10 and then install Windows 7. Then you are going to have to find the chipset, graphics, audio, etc., drivers for Windows 7.
You would be miles ahead if you just purchased a laptop with Windows 7 already installed on it. Staples has a refurbished Lenovo T400 for $189.
This is the best I can do for you, you're on your own.