It is tough to find waterproof pants that are also briar resistant. I got a pair of these to wear under my briar resistant pants this year. They are under $45.
I used to wear jeans under my brush pants once the weather turns colder (shorts under them before). I switched from jeans to the above pants part way through the winter. On warmer days, they did get a little warm from time to time, but they did keep the water off my skin. They breathe pretty well too.
I can't recommend wearing them when it is warmer, but I also get really warm, so it is possible they might not be too warm for others. It is also possible you could wear them without real brush pants over the top. I just don't know how well they'd do at briar protection.
I didn't notice any problems with bending my legs, going over or under fences, etc., so they probably stretch well.
I would rather the zippers were horizontal instead of vertical, but they did stay zipped for me.
Oh, and they do sell them in an orange color, if you want to prevent getting shot in the legs.
Might not be what you are looking for if you want an all-in-one pants, but thought I'd throw them out there. That reminds me, I'll probably order another pair to have a spare.