RenShape is good stuff and so is impregnated mahogany.
MDF will be trouble - depending on your release and your methods. If you are sure the geometry is good and want to make a lasting mold, use something other than MDF. Otherwise, MDF will be fine for a proof mold, maybe. The 'grain' is difficult to seal. You will see the texture on your part surface.
For almost all composites a master is made and the tooling is pulled from the master. This allows to make a non-pourous mold surface from a slightly pourous master.
Scott
Consistency is a good thing....unless you're consistently an idiot.