I sell Macbook duo cores on Craigslist. Most I load Snow Leopard on with a SSD but I dual boot Windows 10 and or a version of Linux. These machines tend to only have 2gb ram and run better with Windows or Linux then OS X. Snow Leopard, Lion and Mountain Lion are no longer supported by Apple. While the hacks can get you to Mavericks or beyond unless its a Macbook Pro I just wouldn't.
Google reFind. Use it and Linux distros like MINT or Ubuntu or Arch and try them out. Take note of CPU and Ram use. Its lower then OS X. If you don't want to go through the hassle of making your Linux install look like OS X use distros like Elementary OS or Zorin or Pear OS. Windows requires extra drivers. Check out the Bootcamp updates or find drivers for sound, etc. on-line. On a budget? These units are around $50.00 on Ebay. A 128 SSD is $40.00 or less. Some have 4gb ram.