INDIA'S finance minister Manmohan Singh is due to present a budget next month that will build on the success that has accompanied the reforms he launched in mid-1991. His principal successes include bringing inflation down from an annual 17 per cent to seven per cent in two years, and increasing exports to sharply cut the trade deficit. Exports are growing by about 20 per cent each year. Furthermore, reserves have soared from US$1 billion in 1991 to $9 billion, according to the National Council of Applied Economic Research. Other estimates put the foreign exchange reserves at $13.75 billion. Mr Singh has reversed four decades of socialism by opening India to foreign investment, cutting the country's protectionist trade barriers, and floating the rupee. To ease foreign investment, four stockbroking firms were permitted to operate in the country for the first time last December. Analysts in Bombay said this would result in a much larger flow of foreign investment into the country. During the previous year, 124 foreign funds had invested $650 million. Mr Singh expects between $1 and $1.5 billion will have flowed into the country from abroad by the time the present financial year ends in March. To encourage trade, Mr Singh has steeply reduced tariffs to 80 per cent. He envisages further reductions, and plans to dismantle import controls entirely. To enable free capital movement, he floated the rupee on the trade account in March 1993. He has promised that the rupee will be fully convertible in ''one or two years, perhaps earlier''. He has said that his goals were to reform the tax system, reduce the country's fiscal deficit, and transform the entire official decision-making process. In the forthcoming budget, scheduled for February 16, he is expected to cut corporate taxes, deregulate interest rates, cut subsidies, further reduce tariffs, expand convertibility of the rupee, and continue the divestment of public sector companies.