(Reuters) - Morgan Stanley agreed to buy the rest of a brokerage joint venture from Citigroup Inc at a lower-than-expected price valuing the business at $13.5 billion, a win for Morgan Stanley although it faces challenges to boost the operation's profits. The brokerage business was meant to stabilize Morgan Stanley's revenue during economic cycles but has not been nearly as profitable as the investment bank had hoped. Costs have run over and technology problems have dogged the joint venture since its inception. ...