I've finally gotten a mold made for my bmw e30 racecar trunk/boot and I am starting to plan out the layers of materials so I can infuse the part.

Are there any general recommendations on how many layers and core material to use? This will be basically just a skin part (I won't be making the inner structure like was done in that mini cooper bonnet video from easy composites) that will be pinned to the car. Obviously the car is only for track use so lightweight is the goal, without being to flimsy.

Thanks for your help!

It's not so much about the stiffness, it's about the weight. Rigidity is largely a function of thickness, so if you had 2 laminates with the exact same layup and thickness but different types of core, then you likely wouldn't notice much difference in rigidity (as far as real world rigidity goes anyway, there will be some differences if you measured them scientifically). Certainly for something like a bootlid, which isn't really a loaded panel, it doesn't really matter what core you use from a structural perspective.

But the weight differences can be huge, relatively speaking. And a lot of it comes down to how much resin uptake the core has. PVC core has quite a high resin uptake. Divinycell isn't bad. Rohacell is the best. I dont personally know Mycell, it is probably a competitor to Divinycell which is the most common. 

Realistically, it isn't going to matter much. When it comes down to it, you're probably talking less than a few hundred grams difference. But Rohacell is the best, so I tend to just recommend it.