Obito has seen Minato in action and looked up to him, I highly doubt Obito would underestimate him.
madara knew tobi. when he was revived( edo tensei), he guessed that everything's probably his doing. i doubt if he ws referring to kabuto. it was tobi. who knows when madara died. what we know is that he lived a long time after his fight with hashi. theres a possibility that he met obito. used him, influenced him, or even train him, whatever. so he became arrogant when he faced minato, thinking that he can take him. with his kamui, he knew that if things go bad, he can always escape with ease, so yeah, why not try fighting minato. theres nothing to loose. anyway, these are just my thoughts. there are so many hidden gaps that counts many possibilities. this is how i see them.