You can clean install a full version provided you have downloaded the full install files, If you only have the update file then you need the previous 7.0-5 version installed.
The Key commands are best explained in the Operation Manual chapter dedicated to this. This will explain how to modify and creat them etc, and also export and import your personalized KCs via XML file. It will also describe most of the default KCs. The Cubase preferences are to be found in your roaming folder.
You can download complete ISO files from the Steinberg site. With those 2 you can install Cubase 7.5 complete, without need to update or have your other disks.
The only thing Cubase cares about, is if you have a 7.5 license on your USB licenser. That means you can use 7.5. How it’s installed, doesn’t matter anymore then