A small amount of synthetic fibers can help stabilize and retain the shape of wool garments without affecting other properties of the fabric. Either way, fabric type won't prevent you from sweating on the bike. Wicking fabrics move the moisture from the skin, which can help keep you more comfortable.
Merino comes in different grades; for base layer you need superfine grade. 100% wool is best for the finest grades. Lower grades of wool need some reinforcement from synthetics.
A plain T shirt style is good for winter.
My first merino T was made from lots of panels stitched together, it wore out at the heavy seams.
My current T shirts are thin, stretchy, flat seamed and have lasted about 4 years of heavy use. They are NZ made and easy to care for.