People overthink too much on Sagat. He is ranked the best among the cast but the game in general is pretty balanced. He is definitely beatable, so pick the character you want(even Dan in this one), dont care what other people say. Whatever is most comfortable is the right choice.
You should look at some of these:
Basics:
YouTube - Street Fighter 4 Tutorials - Ep. 1 - The Combo System
Focus:
YouTube - Street Fighter 4 Tutorials - Ep. 2 - The Focus System
Go to Training Mode, and record the dummy to do Blanka Ball over and over at you. If you block the ball i think you can punish with what i said above. If you spam crouching Light Punches it might be able to beat out Blanka Balls with the right timing, resetting his position. Her Standing Jab however is slower so i don't recommend that if you are spamming it to poke out an anticipated ball. Don't Focus, it breaks right through it.
However if you anticipate a Vertical or Rainbow ball, you can Focus those, then get a free combo after.
From what i read, its an even matchup. You can do it!