I made the transition from an insulated coat to a shell last season. As long as the shell is windproof and waterproof, it's very easy to layer to match your riding conditions. I ALWAYS take an extra layer, but typically begin with a base layer of silk or polypropylene, a mid-layer of fleece, and then the outer layer which is the shell. You can always take off a layer, but if you don't have an extra layer when you need it, it can be kinda dangerous.
I still have insulated upper chest high riding pants, and I'm sure these lend to "core" warmth as well. Probably should take your riding pants into consideration as you equip yourself.
As John has stated emphatically above, stay completely away from cotton. Rode with a couple of guys last winter who had cotton T shirts on as their base layer, and they regretted it in a big way. We got a stuck in an off trail situation, worked like the dickens to get out, and when we did, they quickly got real cold. Ultimately they had to stop, peel off the cold wet cotton, and just used their fleece 2nd and 3rd layers (real cold day) which allowed them to warm up quickly. Good lesson learned for them I'm sure!
Good luck in your decisions.