Depends on what you want to do with it. If you only want to play games, then I recommend:
- stick with 3.1 - you won't find much use for the later OS versions
- "RTC" is real time clock, I think. I thoroughly recommend you get a 4MB RAM expansion, it makes a big difference believe me.
- Probably not worth bothering with USB, although you could transfer files using USB storage devices? Might be useful if that's the only way you can transfer files between your Amiga and your Mac. Then you can download all your games off the net and transfer them to your Amiga, I assume.
- Magic Pack software is the software bundle Escom used to sell the Amiga with (or try to, at any rate). I think it contains 2 games: Whizz (awful) and Pinball Mania (even more awful), and apps like Wordworth and Photogenics.
EDIT: In this case, thinking about it, USB might be a good idea...