A secure wipe is not needed and will waste some of the drives write cycles.I'd do a secure erase (TRIMs the whole drive) on the SSD with gparted, aLinux live CD and then when you've reinstalled Windows disable hibernation (will free up about 5GB on the SSD) and if you have lots of RAM (8 GB+) than disable the pagefile which will free up several more GB.
You can set your Steam folder to the relevant folder on the HDD and it will be able to use the games again (I've done it before). Just make sure you back up your game saves before wiping the SSD. Videos will be fine.
Erasing the free space is fine but by does it need to be done securely? One pass should be enough.
A secure wipe puts it back 100% to factory default. All the cells are completely empty and performance will be 100% restored. A normal format will not do this.
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