I don't think you should get a heavyweight baselayer, unless you buy a lightweight baselayer too. I find it better to get a heavy mid-layer for warmth instead.
For extreme cold, like 2 degrees and heavy winds, I wear a lightweight Sessions Diffusion baselayer top and bottom, and layer with a Special Blend heavyweight 300g fleece top, and then my jacket, which is not insulated, and I'm warm all day. For a middle layer bottom I wear a mid-weight Bakoda pant over my baselayer.
Check out the Northface Denali jacket as a mid-layer option its 300g with good venting, which I've found most layerig fleeces dont have.