In the meantime what's the best way to clean my system before installing ML? I'd like to delete old prefs, duplicate docs, and "defrag" although I believe that's not necessary as of Lion?
Defragging has been unnecessary for much longer than that. Since at least Tiger (2005), if not earlier.
My general advice is that cleaning apps are not to be trusted: they frequently do more harm than good. Second: OS X does not need its oil changed at regular intervals. You do not need to run any periodic programs for "maintenance", other than the ones which the OS does automatically itself.
However, if you want, you can trawl through your user Library folder and delete anything that is no longer needed. Application Support, Caches, and Preferences are a good place to start.
It's worth bearing in mind that any files there are likely to be very small, and if left unattended will do no harm to your Mac. They won't slow it down, or wear it out.
OS X is designed to be installed "in the place" of an existing system, and should leave your data, prefs and apps alone. (But you do have a backup, right?) Doing a clean install achieves very little, particularly as most people immediately follow it by migrating everything back to where it was.
I've installed Panther, Tiger, Leopard, Snow Leopard, Lion and Mountain Lion on several different Macs "in place", and never had a problem as a result.