How do I refresh a "Lion" iMac to run "Mountain Lion" (factory install, wipe drive)
I always do option 3. You should be able to pretty much follow these directions, even though they reference Lion: http://forums.macrumors.com/showthread.php?t=1189888

Long story short: When you buy an app from the app store it is stored on your drive in a format that can be burnt.

Of course, the USB drive option is also doable and it sounds like that would be easier for you.