Don't know about the cocaine bust; Bush has never been OFFICIALLY linked to Cocaine; rumors, but no adminssion or arrest, as far as I know.
But, to answer your question, could a black man be elected president if he had once been pulled over for DWI, and had rumors of past Cocaine/drug use?
Yes.
Obama, and Powell are two of the most respected MEN in this country right now. Conservatives LOVE Clarence Thomas; we currently have a female, black Secretary of State AND a female Speaker of the House (what's that 3rd and 4th in line?)
There isn't enough racism left in this country to keep a black man from getting elected president just for being black, IMO.
Now does that mean Jesse Jackson can run and get elected? No. But Obama can....and so could Powell.