On the Macintosh system software: 7.5.3 (and an updater to 7.5.5), 7.0 (and an updater to 7.0.1), and 6.0.x are available from Apple. The general rule is that older versions use less RAM, less disk space, and are faster. If you want to run generic Mac software 6.0.8 is fine for older games, back when they still developed games for the Mac. System 7.1 is the minimum for most applications, though you may get away with 7.0.1, simply because the system 7 era lasted the longest and application development was done throughout.
Question: are there any good Apple II emulators for the Amiga?