Stuck in recovery mode and unable to install OS X El Capitan
Everything you said tells me "hard drive failure." It is not uncommon for a system to have boot issues when the hard drive is failing. The fact that you can't see the other partition and "this item is temporarily unavailable makes it sound like that's the issue. 

Often when you try to reinstall the OS with a failing hard drive, you will get errors. This is primarily due to the hard drive having to write a file (the new OS) to the entire hard drive, whereas if you're using it, you're only accessing specific parts of the hard drive. You may have been avoiding that one "failing" spot, but when reinstalling the OS, it hit it.

I'd highly recommend taking it to an Apple Store or AASP (Apple Authorized Service Provider) and have them run a diagnostics check on it to verify. Or if you have an external bootable usb drive, attempt to boot into that.