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Cheers - I did end up installing eOS with a lot of ubuntu-based mods, but the end result was that it didn't perform as well on the aging apple hardware as I thought it would (slower, processor overheating even with tlp and a fan daemon installed). To be fair, I didn't meet the minimum requirements specified on the website (i3 minimum vs my core2duo) so that was probably it. Back to the insecure snow leopard for me.
The distro you choose will have minimal influence on what you can actually run. Libre office will run on pretty much any linux distro.
I haven't used ElementaryOS, but as far as I can tell it's just Ubuntu with a different desktop environment (Pantheon - which is based on Gnome)
I'd definitely recommend Ubuntu - it has by far the largest user base and the best community support. In my experience it requires the least tinkering to get working, and when things go wrong, it's the easiest to find a solution for.