A wealthy businessman will always be considered a serious candidate. They've demonstrated that they are able, capable, and successful already.
I wouldn't call Trump a truly "serious" contender though.
He was #1 in the most recent poll and is risking/losing millions upon millions of personal wealth. Seems serious enough to me. With him, there should be a distinction made between being manic and not being serious. He's completely serious but he's also off the wall and a megalomaniac.
Just to expound on my GOP brand comment. Thats just a talking point by establishment republicans and scared opponents. Its somewhat valid from the Dem point of view if he was to make it all the way..however, it is livening up the GOP which would otherwise be incredibly bland...will probably result in higher turnout for or against him..and he can actually help the GOP brand by being so far right and thus making the rest of them seem more reasonable. The R's know this but wont admit it and are just disowning him. Thats the proper strategy by his opponents but it is not accurate in terms of the eyes of voters. The GOP brand couldnt be any worse as it is and Trump's main appeal is that he is an outsider that doesnt give a fukk and is far from the general establishment of the party anyway.