I've found if I wear cuddlduds (idk if I spelled that right...) under my regular breeches or even my summer weight kerrits tights i'm pretty comfortable temperature wise. Just a thought should you not want to spend a lot of $.
Yes, they are full seat so if you don't like full seat, you won't like them. On a somewhat green 5yr old who is angel one minute and a spazzy nightmare the next, I LOVE the sticky seat.
Kerrits makes long underwear to wear under breeches. It's knee length so it doesn't interfere with the fit of tall boots and although they seem to be paper thin, extremely warm!! Between the long underwear and their breeches, I was warm and comfy at -3 last year.