I was hoping to get a recommendation on the type of plastic to use as a mould/ baseboard.

I'm making a few large panels (about 1700 x 600) via resin infusion. The panels are mostly flat with a few features, e.g. one has small fold along the middle and curve near the edge, one panel needs a gentle dome in the centre.  I need a baseboard plastic that:
 - is heat formable, ideally with a heat gun
 - leaves a great surface finish
 - is easy to release, and
 - inexpensive

Was thinking maybe 3mm polypropylene glued to some 12mm MDF to give it some dimensional stability and ensure it stays flat under vacuum.  There are so many types of plastic I'm sure some are more suited to the task than others.

Will I still need to wax and/or use a release agent?

Polypropylene is going to work for your plans, I just wanted to make sure that you are aware that you wont be able to use a gel-coat on the PP film as the surface tension of the PP is so low, that the resin won't wet the surface evenly.

cheers, Reto