UNDERTAKING its biggest peacekeeping job ever, the United Nations is scrambling to keep pace with a changing Cambodia.A 300-man vanguard of what will be a massive UN team of administrators and peacekeeping troops is filtering into Cambodia's capital. Eventually expected to number more than 10,000 and spend $1.5 billion in two years, the UN corps will patrol a cease-fire, disarm and regroup the armies of the four rival factions, resettle a half million refugees, run a transitional administration, and supervise free elections by early 1993. Yet, with preparations frozen until a formal settlement was initialed in Paris on Oct. 23, UN officials admit they are now struggling to get under way. "Everyone had hoped for peace in Cambodia," a UN official says. "But when it finally came, we were all caught with our pants down." Bogged down by politics, bureaucracy, financial uncertainty, and the sheer magnitude of its Cambodian task, the UN itself is the center of controversy. In a world overloaded with calls for humanitarian aid this year, no one is sure where the money will come from. The UN High Commissioner for Refugees has yet to raise the $33 million it needs to begin repatriating refugees in camps along the Thai border. More than $100 million will be needed to complete the resettlement. "It's all possible if the donors respond quickly," a UN official says. The scope of UN powers also is unclear. Among other issues, the peace accord is vague about how UN will coordinate policy with the Supreme National Council, an interim body comprised of factional politicians and headed by the returned former monarch, Prince Norodom Sihanouk. That uncertainty has been most glaring in defining the role of UN peacekeeping troops charged with demobilizing two-thirds of the estimated 180,000 rival troops, including the more than 20,000 disciplined, jungle-based guerrillas of the Khmer Rouge. For example, UN officials recently indicated that if their lightly armed troops encountered a cease-fire violation, they would report it to the UN Security Council and await instructions. "The communications problems alone will be huge," says an officer wearing the blue beret of UN peacekeepers. "It's going to be very, very difficult." The country's vast carpet of land mines, which daily kill and maim scores of Cambodians, are the No. 1 enemy, UN officials and international aid workers say. Yet, the first group of UN officials only carry the mandate for mine awareness education. "Everyone talks about mines in Cambodia, but no one does anything about them," says Jean-Jacques Fresard of the International Committee of the Red Cross, who fears disarmament might trigger new mining as the factions try to fortify strongholds. "You won't find any foreign country sending anyone here to de-mine. It's far too dangerous." Although the UN worries about too little funding for Cambodia's transition to peace, officials of UN agencies and international relief organizations say too much money, at least initially, may be allocated improperly. While homes, schools, and hospitals need to be rebuilt and transportation and communications need to be restored, assistance will have to be carefully targeted and monitored to be effective, observers say. "The infrastructure is very fragile," says a senior UN official. "It's not just a question of the amount of aid. It's also a question of the absorption capacity." Despite calls to channel assistance to the poorer countryside, UN officials admit the impact of the huge international assistance program will be seen first in the cities. That worries officials who say rural disgruntlement could be tapped by the Khmer Rouge. "It will be a very long time before projects are in place," a UN official says. "There will be a lot of money spent in Cambodia. Initially, it won't be showing up in the countryside, but in Phnom Penh." UN officials handling the return of refugees say that repatriation also will be touchy and difficult. In its six border camps in Thailand, the UN has played taped speeches from Sihanouk, urging Cambodians to wait for UN resettlement next April. Still, 100,000 of the 350,000 Cambodians on the border are expected to come home on their own, straining the supply of available, un-mined land and an agricultural sector battered by widespread flooding this year. Flood damage will cause a food-production shortage in 1991 to almost double next year, the UN says. The UN must also keep an eye on the more than 170,000 internal refugees and other Cambodians who endured their country's turmoil at home. "We have to be careful how we balance what we give the returning refugees and what we give people here," says Bjorn Johansson, a UN refugee official. But the biggest threat to a smooth resettlement are land mines, estimated to number up to 4 million. Hard-to-detect plastic mines, sown haphazardly in dense jungles and rice paddies, have already stopped the return of many internal refugees whom the UN had hoped to resettle before the border Cambodians arrive in April. De-mining begins in May. Realistically, Western aid workers and military experts say, the UN will have to try to clear a few corridors and seal off large areas with warning signs until Cambodians can be trained to do the mine-removal themselves. "Mines are scattered willy-nilly along jungle paths where they are the only entry and exit to areas," says a military analyst whose country helped Cambodian resistence groups in laying the mines. "Clearing them will be very dangerous and highly time-consuming."