The leader of the political coalition allied with El Salvador's leftist guerrillas said yesterday that he would be happy to meet with newly named U.S. Central American envoy Richard B. Stone even though Stone "is not the ideal man" for the job.

"If Stone asked, yes, I would talk to him," said Guillermo Ungo, president of the Democratic Revolutionary Front. "I believe Mr. Reagan is not the ideal president for us to deal with. Mr. Stone is not the ideal man, but we have to see."

At a luncheon meeting with Washington Post editors and reporters, Ungo and a Washington-based aide also said that the opposition would seek in any negotiations to purge the Salvadoran Army of "killers and Nazi Salvadorans" and to more effectively implement land reform.

Ungo said he doubted that the United States' Latin American allies would give the same support to planned elections later this year as they gave to last year's U.S.-sponsored vote, and he acknowledged that the recent deaths of two rebel leaders were "a very important loss."

Under heavy pressure from Congress, Reagan named Stone Thursday as a special envoy to seek a negotiated settlement of the Salvadoran civil war. His nomination must be approved by the Senate, where he faces criticism for his lack of diplomatic experience, conservative views and past work for Guatemala's rightist military government.

Some congressional leaders have suggested that Stone was a poor choice because he would have difficulty winning the trust of El Salvador's left. Ungo expressed skepticism that the U.S. administration was serious about promoting negotiations but said that he was willing to give it a chance.

"We believe this Stone's nomination is a kind of window dressing. The administration always has refused to negotiate; it acted only under pressure from Maryland Democratic Rep. Clarence Long, from Congress," Ungo said.

"His Stone's functions are unclear, but I would keep it open the possibility of talks . Mass media, public opinion and Congress may put pressure on the president."

Stone's relations with Ungo, a one-time running mate with former Salvadoran president Napoleon Duarte and the top figure in the loosely structured opposition coalition of politicians and guerrillas, would be crucial to achieving a Salvadoran settlement. Ungo's front, which has its headquarters in Mexico and is known by its Spanish acronym FDR, effectively is the political representative of the guerrillas of the Farabundo Marti National Liberation Front, or FMLN, which is battling El Salvador's U.S.-backed government.

The Salvadoran opposition in October called for talks with the government without preconditions regarding the subjects to be discussed, but so far the Salvadoran government and their U.S. allies have rejected the proposal, on the grounds that the only subject for talks should be planning elections.

Ungo appeared unwilling to specify what the opposition would seek to discuss in talks, saying that the agenda itself should be a topic and emphasizing his view that far-right Salvadoran leaders were blocking negotiations.

Under pointed questioning, he and Alberto Arene, a representative here of the FDR-FMLN coalition, said that the agenda should include establishing peace, encouraging democracy, "cleaning" the Army of extremist elements and replacing them with rebel units, and expediting agrarian and other reforms that he charged the far right was trying to block.

Comparing negotiations to a soccer game, he said: "All 11 players on the left are willing in an agreement to go that way toward talks , although some are less enthusiastic. The problem is the other side, where three or four don't want to play, and they have guns."

Ungo said that the diplomatic climate has changed since last March's Salvadoran elections and that, therefore, the United States should not expect to find widespread support for another election scheduled in December. He noted that Mexico, Venezuela, Colombia and Panama already have launched a diplomatic initiative on Central America independently of Washington and suggested that these nations and possibly U.S. allies in Western Europe were less willing than a year ago to endorse U.S. policies in El Salvador.

Asked about the suicide last month of Salvador Cayetano Carpio, leader of the FMLN's oldest component group, Ungo said that it "gives ground for more and good propaganda against us" but stressed that Carpio's faction would survive because it is "grown up."

Carpio killed himself after learning that a close associate had plotted the murder of his second-in-command, Melida Anaya Montes. He said that Anaya Montes was killed for both personal and political reasons but that he tended to think that it was "more a matter of personal reasons, in a small group."