I'm trying to make the Application Launcher appear when I press the Windows key but it seems like I have to enter at least two keys to make a shortcut (currently doing Win+space)?
I use the "menu-key" start Synapse. You could try to bind that to the Application Launcher. The "windows-key" will probably not work, because it is a modifier-key and always used in conjunction with another key.
I don't think that this is possible in terms of system default, because you usually use single keys in a different way.
One option would be though, to work around this with autokey or anything other able to handle hotkeys.