It will take 2 months to gather the actual requirements and be able to do an estimate.
This guy consults.
From my experience, the two months of gathering requirements will be more valuable than developing something from zero requirements. Whatever you code with always be "wrong" because with zero requirements there is nothing to test against.
"It will take me X weeks... to come up with a reasonably accurate estimate."
"This is git. Have fun!"
Fork an existing one, slap your name and logo on it. 2 points.