The head of El Salvador's armed forces has a military wish list that could easily cost the United States half a billion dollars, a visiting U.S. congressman said today. Two colleagues agreed on the content of the list without specifying the figure.

Reps. Thomas Harkin (D-Iowa), James Oberstar (D-Minn.), and James K. Coyne (R-Pa.), ending a three-day fact-finding visit, said at a press conference that the defense minister, Col. Jose Guillermo Garcia, had told them he would need much more military aid to crush rebel guerrilla forces.

Garcia's list, Harkin said, included "enough small helicopters to move two battalions at the same time," or about 180 of them; several large C123 transport planes; 10 A37 jets, some small patrol boats and electronic-surveillance equipment. Garcia said he did not need all the aid at once, Harkin added.

Harkin said in an earlier interview that the total cost of such equipment would be "easily half a billion dollars." Congress is opening a highly charged debate on a proposed $60 million in military aid for El Salvador for the next fiscal year, more than doubling the $26 million allocated in the current budget. The United States also is supplying $55 million in helicopters and other military equipment in a supplemental emergency package.

Harkin said he had seen nothing during his visit to change the view he formed in a visit two years ago, when he strongly opposed any U.S. assistance. The armed forces here, he said, have been "corrupt to the core for 50 years," and if they receive the aid Garcia asks, "they would go on a rampage like you've never seen . . . It could be a genocidal type of thing."

U.S. Embassy spokesman Howard Lane said he had never heard of a request for $500 million in aid. "The Salvadorans have their shopping list, which is subject to discussion," he said.

The three representatives complained that they could not get the truth about the human rights situation from Salvadoran military officials. They expressed particular concern about the deaths of 27 persons Jan. 31 in a poor San Salvador suburb, San Antonio Abad.

"It's a massive cover-up by the military," Harkin said. He quoted Garcia as saying 27 young people had been killed in cross fire when 40 National Guard soldiers were fighting guerrillas there. "But the embassy people said they found people with bullet holes in the back of their heads and with powder burns," Harkin said.

Coyne said one woman had told him her two sons had been taken away at 3 a.m. "by men who said they were looking for snakes." She found them in the street the next day, Coyne related.

The freshman Republican said he had decided that "a great chasm" exists between the military establishment here and the civilian junta headed by President Jose Napoleon Duarte. "It's not like the United States, where the government and the military are more or less the same," he said. "I'm not sure President Reagan understands that."

Oberstar said he came away with doubts that elections scheduled March 28 will prove much, since so many Salvadorans have fled the country or are refugees who cannot vote in their home towns as the law requires. "The test of democracy will be what happens here after the elections, if the government can get control of the situation," he said.

In an earlier interview, Oberstar said Garcia had given tentative approval to the idea of a international force attempting to help stop the flow of arms, if any, from Nicaragua or Cuba into El Salvador. Oberstar told the press conference that patrol boats could watch the coastline while international troops patrolled the 300 miles of rugged frontier along the Honduran border. "This is an option to be explored, and surely it would be less costly than what we are doing now," he said.

Harkin said the U.S. Embassy had tried to discourage the group from coming by cabling him that it would be difficult to provide adequate security. "They were simply following orders from Washington, which didn't want us to come," Harkin said. The Boston-based Unitarian-Universalist Service Committee paid for the congressmen's trip.

Embassy personnel said privately that they saw no point to the visit of congressmen whose minds were already made up. They have been working long hours on itineraries.

Sens. Claiborne Pell (D-R.I.) and Patrick Leahy (D-Vt.) said on leaving El Salvador last week that Garcia and other government officials had not been honest with them, and outraged editorials followed in the local press. Gen. Wallace H. Nutting, commander of the U.S. Southern Command, had 48 hours in which to see assorted officials, and Rep. Clarence D. Long (D-Md.) is here now.