Win8 works fine - as does Win7. Just install it. Buy a retail version (a "System Builder" version is fine) and install from DVD or use one of the many tools to copy a DVD to a bootable USB drive. You will need some drivers which you can still find at the Dell site - see various posts in
Windows 7 Guides
If you choose Win8, you'll probably want Start8 or one of the other ways of restoring the regular Start menu. Win 8.1 works too - that's what I'm running now.