Outgoing Montgomery County Executive Charles W. Gilchrist, who says he is leaving a legacy of lower property tax rates and more services for the needy, proposed yesterday a budget for fiscal 1987 that tops $1 billion for the first time.

The proposed operating budget of $1.046 billion, which the seven-member County Council must adopt or modify by May 15, represents a 7.4 percent increase for the county of 633,000 residents. It includes a $20.9 million surplus to buffer against expected federal budget cuts under the Gramm-Rudman-Hollings deficit reduction measure and possible state budget cuts.

Although Gilchrist said his budget would effectively lower property taxes, most county homeowners -- about 60 percent -- will pay an additional $25 in property taxes next year because of the rising value of property. Additional municipal taxes will increase costs for property owners in Rockville by about $46, and in Damascus and some other upper county areas by $161.

Gilchrist said Montgomery's tax rate now is $3.14 per $100 of assessed value, compared with $3.79 per $100 in 1979.

With the proposed operating budget, Montgomery joins an elite corps. Fewer than 50 of the nation's 3,000 counties have billion-dollar budgets, according to Susan White, a lobbyist for the National Association of Counties.

At a time when other politicians are coping with budget cuts and scrambling to explain decreasing services, Gilchrist boasted yesterday about the county budget. "I wish I were running for reelection on this budget -- I can't think of a budget I'd rather run on," he said. The 49-year-old county executive, who has held the position for the past seven years, plans to begin training for the Episcopal priesthood after he leaves office on Dec. 1.

The operating budget pays the salaries of county workers from teachers to maintenance workers, and pays for schools, libraries, social services and enforcement of county ordinances such as building codes. It complements a $1.24 billion, six-year building program, the capital improvements budget, that Gilchrist proposed for the county in January.

The largest single increase Gilchrist proposed was for the county Board of Education, which would receive $472.6 million, up 8.1 percent from the previous year. Superintendent Wilmer S. Cody said through a spokesman that he plans to push in upcoming budget hearings for an additional $5 million sought by the board.

Montgomery College faces a revenue gap of $600,000 under Gilchrist's proposed budget, which gives the college $20.8 million. He said a tuition increase of 5 percent would close the gap, and he added that he had asked the state for additional funds for the college.

The proposed budget adds $4 million to the county's human services coffers, with the increases divided among residents with special needs, children, the elderly and housing.

For the first time since he reorganized the county government and created four new agencies in 1979, Gilchrist is proposing creation of a new agency, a $1.9 million Department of Alcohol, Drug Abuse and Mental Health. The budget proposal includes 15 new firefighter positions to offset a decrease in volunteers, and slots for 23 more police officers, Gilchrist said.

His proposal also calls for contracting out such tasks as pruning trees, mowing grass and spraying pesticides on county property, as well as replacing lights and increasing parking violation enforcement.

In Maryland, only Baltimore City's $1.22 billion budget exceeds Montgomery's proposed budget, according to county officials, who noted that the city has 114,000 more residents. Ten states have operating budgets of less than $1 billion, according to Karen Benker, a spokeswoman with the National Association of State Budget Officers.

With average per capita income in 1985 of $23,025, Montgomery County has been more insulated from federal budget slashing than most of the nation's counties. "The disparities that exist between a Montgomery County and, say, a small county in rural Texas are enormous," said White. "What we're very concerned about, for local government and even healthy counties like Montgomery, is we're facing an elimination of general revenue sharing -- the only program in the federal budget that goes directly to local governments to spend as they see fit," White said. PROPOSED FY 1987 BUDGET HIGHLIGHTS

*The Board of Education to get the biggest increase in spending, rising 8.1 percent to $472.6 million.

*The police department to get $707,870 for added patrol strength.

*Fifteen positions to be added to the fire department.

*$1.9 million to be used to create a Department of Alcohol, Drug Abuse and Mental Health.

*$500,000 to be used for a new senior center and to expand services for the elderly.

*Urban areas to receive $1.5 million for increased maintenance.

*Libraries to receive funds to increase book buying, open a new storefront branch in Germantown and improve the computer circulation system.

*Eight construction code enforcement employes to be added.

*$1.4 million to be spent for Ride-On bus service along the I-270 and Rte. 29 corridors.