By Ju-min Park and Tony Munroe

SEOUL (Reuters) - United Nations Secretary-General Ban Ki-moon is the odds-on favorite to be the next president of South Korea - if he wants the job - thanks to high name recognition, a clean reputation and what is seen to be a lackluster field of rivals.

But vying for the December 2017 election would cast Ban into a political arena far more bruising than the genteel give-and-take of global diplomacy to which he is accustomed, exposing his family, finances and career to intense scrutiny.

Ban is coy about his plans.

He told Reuters on Friday that he will decide on his future when he returns to South Korea in January after his U.N. tenure finishes at the end of the year.

But in his clearest statement yet on his intentions, he said he was conscious of "expectations from many people in Korea that I should make myself available for a better future of Korea."

With the pride Koreans take in him as "president of the world" and absence thus far of strong competition, opinion polls show Ban as the front-runner for the election next year.

However, his lack of a political base, a decade-long absence from South Korea to lead the United Nations, and his age - he is 72 - are potential liabilities.

Kim Chong-in, former leader of the opposition Democratic Party of Korea, said Ban many not be able to run a rigorous campaign after having been away for so long.

"In this day and age when the world changes so quickly and people's way of thinking changes dramatically, is it going to be easy for him to even grasp the reality here?" Kim said.

Some U.N. diplomats say the gentlemanly Ban has been a weak leader.

"He hasn't had the ability to really drive international opinion on any of these big issues and he's tried to do the job as sort of best friends of every member state," said a senior U.N. diplomat, speaking on condition of anonymity.

"You can't do that when there's a morally right position and a morally wrong position."

Ban rejects the characterization that he is not a strong leader, arguing that he has stood up against injustice and violations of human rights and spoken up more strongly than other world leaders for the common good.

He said there was a misperception that a charismatic style was needed for effective leadership, and that there were "serious misunderstandings" about his leadership style.

"There are many different leadership styles," Ban said in an interview in his office at U.N. headquarters in New York.

FRAGMENTED FIELD

Ban, a career diplomat who served as South Korea's foreign minister from 2004 to 2006, has never held elected office. He is expected to join the conservative Saenuri Party of President Park Geun-hye if he runs for president.

A complicating factor emerged on Monday when Park proposed amending the constitution to scrap the five-year single-term presidency in favor of two four-year terms, which would make Ban a less attractive candidate given his age.

Ban is the only potential candidate consistently polling above 20 percent.

Former software mogul Ahn Cheol-soo, considered a reformist who would inject fresh ideas and promote ethical politics, has seen his popularity dwindle after members of the People's Party that he founded were implicated in illegal campaign financing.

Opposition leader Moon Jae-in, who would be Ban's main opponent, has an 18 percent popularity rating in polls. But he has been at the center of a controversy over revelations that he supported abstaining from a U.N. vote on North Korean human rights.

Ban's story of bootstrapping success is the subject of near-myth in Korea.

Born in a farming village in Eumseong County in the central part of the country, he was the eldest of six children and grew up poor but became enamored of the English language and dreamed of becoming a diplomat.

In high school, he was chosen to participate in the Red Cross' VISTA (Volunteers in Service to America) program, meeting President John F. Kennedy on a visit to the White House.

Ban is seen to have been laying groundwork for a run since a visit to South Korea in May, when he met political heavyweights including former Prime Minister Kim Jong-pil.

According to polling firm Gallup, Ban's support base is expected primarily to be voters 40 and older, the same powerful bloc that was core to Park's election as president in 2012.

But some Saenuri members say the party should choose a candidate who, unlike Ban, has openly opposed the unpopular outgoing president. Park's tenure - like that of many before her - has been buffeted by scandal.

Han Seung-soo, a former prime minister whom Ban has called "my mentor and forever boss," told Reuters that Ban is a "trustworthy public servant" with a clean record that should help him through the bruising scrutiny of presidential politics.

A former South Korean diplomat who served with him described Ban as conscientious and hardworking. But Ban was also careful not to make enemies and was overly cautious and meticulous, not qualities that would suit a country's leader, the diplomat said.

Park Jie-won, leader of the opposition People's Party, questioned whether Ban had what it takes to be a problem-solving leader who sets the direction for the country.

"Ban hasn't been vetted publicly yet," he said. "He may get support and be highly spoken of as U.N. secretary general, but joining local politics is something different."

(Additional reporting by Jack Kim in Seoul and Jean Yoon and Michelle Nichols at the United Nations; Editing by Raju Gopalakrishnan)