I agree with Alan. I would buy mattress first, then sleeping bag. The mattress to stop the cold coming up from the ground (and to sleep comfortably of course) and then to choose between sleeping bag, or sleeping quilt. For my part, I use a sleeping quilt.
As regard thermals, I use them, my top is one of my two baselayers which I carry. My clean clothes changing point on each day is in the early evening after I pitch my tent and after my shower. My 'thermal' bottoms are a cosy comfortable and loose pair of 'fluffy' trousers from Decathlon, £7.99.
I've also learned that the heat retaining qualities a sleeping bag or quilt can be improved by using the same method that I use for keeping warm in the day, layers! I nowadays always carry a 3' x 3' square of Decathlon blanket, (£5.99, the same 'fluffy material as my 'thermal' bottoms) with me just to throw over my torso if it's really cold, my lower leg and feet area is fine. The blanket works either on the top of my quilt or between me and the quilt. When I can find one I'm going to get a 3' x 3' down blanket to replace it.