You can use MacOS up to version 8.1 officially. I got it to run 8.0 but 8.1 doesn't fully boot. Probably some config mistake I made.
You can download version 7.5.3 on the Apple site for free and update that to 7.5.5. Newer version you have to buy. If you do so make sure it is a generic version and not for one model specific!