Second hand laptop with Core 2 Duo (at least) for R500-R1000 will serve well. Not easy to get one with 4GB RAM for that price, but Win10 will run on 2GB too. Just check for maximum supported RAM, so you can upgrade in future. You can get quality business range laptop with nice hi-res LCD for that price.
These laptops might not be qualified by Microsoft for official Windows 10 upgrade, but all premium brands come with Windows XP or Vista licence in BIOS, so Microsoft already got its money and will tolerate the following:
1. Install fresh copy of Windows 7 SP1 (Pro or Ultimate) without entering serial numbers. ISO image is available from Microsoft Web site. Do not connect to the Internet yet.
2. Install "Windows 7 DAZ loader v2.2.2" and certify copy online on the Microsoft Genuine Advantage Website. Note that Google search engine has removed all legitimate hits to DAZ Loader (leaving only links to the 'improved' or malware infected copies). Use alternate search engine, but it doesn't matter, really. This installation of Windows will only serve one purpose - upgrading to the Microsoft's approved copy of Windows 10. When using malware-infected copy of DAZ, disconnect all other computers from the LAN! Don't bother with installing all drivers either. You need just chipset, basic graphics and network drivers for Internet access.
3. Download copy of Windows 10 ISO and use "genuineticket" method to activate Windows 10 on this laptop. Google for details or ask here.
Note: The easiest way to download ISO copy of Windows 10 is to use Windows Vista or older, or non-windows machine. Hard way is when using Windows 7 or newer - use Media Creation Tool with the option "Download copy to use on different machine". Chose 32-bit or 64 bit (matching your copy of Windows 7) and then save on the hard drive as ISO image.
