In current version of Mathematica these keyboard shortcuts are quite handy
Ctrl+K completes current command
GraphPl -> press Ctrl+K -&
I don't believe there is any included function to auto-complete a string. I also cannot recall a way to view all valid settings for a particular option, other than searching the help files.
You can expedite input with the Options Inspector settings InputAliases and InputAutoReplacements, allowing entry by EsctxtEsc or txtSpace.