Hi,
If you have a 3.0 system, then you can get up to 3.1.3 for free, which you can download at the dowload center.
The system requirements are on Avid’s website:
https://www.avid.com/products/Media-Composer-Software/system-requirements.asp.
They are for MC 4.0.2/4.0.3 (the current version of the software).
They have specifically tested some iMacs as well. I’ve ran multiple revisions of MC3 and 4 on an unsupported 2.16 GHz 2006 (black) MacBook, and it runs fine.
Not sure where you can find the requirements (and tested configs) for previous versions of the software. The biggest caveat when you are buying a new Mac today, is that these mostly come with Snow Leopard installed, and Snow Leopard is not supported (yet). That said, a lot of folks are making this work, see https://community.avid.com/forums/p/76313/426540.aspx#426540.
You should always stick – religiously – to the Version Matrix. Using newer/older versions of the Mac OS and/or Quicktime often result in unpredictable errors.