will this (OS X Mountain Lion) install on an iMac mini? I get the message it won't when i tried to buy it

You computers serial number will tell you what you need to know. Click the "apple / about this mac / more info"

Check your computers model and year - https://selfsolve.apple.com/agreementWarrantyDynamic.do.

Once you know the model and year, ensure that your model is supported for the 10.8 OS - http://www.apple.com/osx/specs/.

There's no such thing as an iMac Mini. If you have the Mac Mini Server (late 2012) you claim in your profile, you're already running Mountain Lion.

Matt