I think no, you have not made an intelligent, informed decision. Not once did you say you were in love with him. He proposes to you from a mental health bed and you say yes, not even being in a serious realtionship with him? You said you dated in the past, I guess things weren't exactly marriage material?
He is sick, hon. He doesn't even know what he is saying; he isn't in his right mind. Just because he is down from his mania, he is still very confused. You really need to read up on REAL bipolar, you have no idea what you are in for.