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 excellent as it does have a good surface finish and is fairly easy to shape and work with, it does meet the criterias you are after. Securing it to MDF is also a good idea, so I think you've got it fairly sussed. You may just wish to fill the features of the polypropylene from the back with an expanding foam, just to ensure that the vacuum doesn't distort their shape slightly. They sound like rigid geometry, but you can never be too careful if you are trying to be accurate.

Polypropylene shouldn't need any release agent, but if it were me I would still apply a wax or chemical release agent, just because infusion is an aggressive process and while it isn't likely the resin will actually bond to the plastic, having a release agent can make demoulding just that much easier.