Is it a regular or enhanced?

If it's an enhanced System 6.0.8 will work fine. If it's a regular 512K, you'll need to find an older version of the system. I'd recommend 3.2 for the 512K as it was stable and had a fair amount of features for a 1986 operating system. Apple doesn't make it available so you're going to be on a bit of a floppy hunt, thankfully those old disks do show up on eBay now and then.