My standard for below freezing weather is an Under Armor base with neck collar, 1 light thermal, 1 medium thermal, and a nylon shelled, fleece lined jacket with neck collar. As my core warms and releases blood flow to my extremities, I regulate by unzipping the jacket and shedding the watch cap/gloves as necessary. Also wear a thermal base under either jeans or running pants and heavy wool socks. This works well down to about 22 degrees, which is the lowest I've ridden in.

On tour, I'd take two each of the base layers and pack all the spare clothes, including the jacket, into the rear panniers.

BTW, I'd trade my Under Armor for your merino wool base if it has a neck collar.