What you'll need to do is hook up to the TimeCapsule and let TimeMachine make a backup of your computer. If something doesn't work with the Mountain Lion install, you can wipe the drive and rebuild from the TM backup.
An ethernet cable connected between one of the 1-3 ports of the TC and the iMac will speed up the transfer quite a bit.
BTW it appears that the optical drive in the one machine is failing, reads some discs, not others, seen it before.
I guess my question is what are you using for wireless now? Why not just hook the TC up to your internet and use it for wireless, also it will be a constant TimeCapsule backup. You can back up both iMacs to it.
I have to head out for a few hours, I'll check back in later.
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