Re-image Macs in Bulk and administer them

I've recently been called in to deal with the IT infrastructure of a small non-profit organisation that is going through a restructure that involves purging all its staff and employing new ones from scratch. They have 6+ Mac Minis, an iMac, a MacBook, a MacBook Pro and a Mac server (not a Mac Pro, a silver desktop box).

One of the things I would like to do is take backups of hard drive content, then re-image them with 10.8 and reinstall Microsoft Office 2011.

My research seems to indicate that I need to press ⌘ + R at the BIOS screen, but then other places indicate this this will keep files.

With 10+ (I'm still finding computers in cupboards, the old staff did not keep an asset register) computers, downloading the OS 10+ times seems slow and useless.

I have also seen indications that this will only download the version that came as OEM, which means I would need to update them as well.

How can I re-image all these Macs in a sane way? Coming from a Linux background, all this seems overly complicated. Bonus points if someone can point me to some free software that will help me administer all of these (I watched Google's talk on the matter, but there didn't seem to be anything out of the box).