No arduino is suitable for your needs. - they're microcontrollers, not dsp's .
Not nearly fast or accurate enough.
Nearly all you require could be done by some fairly simple analog electronics.
Sustain is a bit more tricky.
I've sometimes wondered whether sustain could be achieved other than acoustically by providing gain between one guitar pickup and another used as a driver. Or even a single pickup in a Colpitts type oscillator...
The string is the resonant element.
The electronics could either be in the guitar, perhaps powered phantomly, or in the front end of the remote amplifier.
Never tried it. I'm a trumpet player.....
Any ideas, electronically competent guitar players? Given enough gain, something's gotta hoot.....
There goes my patent!
Allan