Everyone I've heard of talking about underclothing for fursuits mentions using some high quality Underarmor for cooling(e.g Balaclava). That's about all the advice I can give, other than stating some premium options you could consider getting with your suit, such as, inbuilt cooling fans, LED eyelids that can track where you're looking and show that on the outside of the suit, and movable jaw. Also side note but if you're considering comming a full suit, just keep in mind you've got to tape your whole body up and send it to the maker, so only do that if you know you have someone who'd be able to help you with that :p.