The one thing to remember if you wipe the machine and install Mountain Lion is that you won't have the apps that come with a new iMac. ML doesn't install iMove, iPhoto, or iDVD. You can purchase all of them through the App Store, iPhoto is only $15 US. If you need iMovie or iDVD to process your video from a video camera those are able to be purchased too.

If you have the password for one user that is an admin you can delete the other users and set up a new user for yourself as admin. Then you can boot to that new user and delete the other admin.

Boot Camp will allow you to delete and resize the Windows partition so that it adds the Windows partition back to the main HD.