About 75 new standard-size buses will be purchased for the Metrobus fleet, Metro officials said yesterday. The buses, which could be on the streets late this year, will be paid for primarily with a newly approved $10 million federal grant.

The new vehicles, which will cost about $165,000 each, will have extra-wide interiors, wheelchair lifts, and, to answer long-standing complaints from riders of Metro's newer buses, windows that open.

But officials said the buses do not guarantee a solution to the system's reliability woes: Federal rules that require Metro to buy from the lowest bidder could result in purchase of unproven or known problem buses or ones with parts and maintenance needs that hinder efforts toward standardization.

"It is a risk," said Metro operations chief Theodore Weigle. But he said he expected the new buses would "generally improve" reliability.

Metro got its last new buses three years ago. Officials said they hope the new purchase will complement a plan to rebuild 600 aging but reliable General Motors buses--the major element of long-range efforts to modernize the fleet.

The purchase will be financed by a $10 million grant from the Urban Mass Transportation Administration and $2.5 million that Metro has earned by investing capital funds, officials said. Metro also expects 32 more "articulated" buses--the elongated vehicles that bend in the middle--to arrive this June. Those buses were ordered in 1981.

Metro now puts about 1,750 buses on the street at peak hours. The system has suffered escalating rates of breakdowns and missed trips in recent years, leading to concerted efforts to tighten maintenance. Metro officials report bus performance has shown encouraging gains in past months.

Metro has found out the hard way that newer buses are not necessarily more reliable. The 43 articulated buses it now operates, made by the West German company M.A.N., and its 113 General Motors RTS-II "advanced design" buses, the sleek white models with tinted windows, have broken down frequently. Both types were delivered in 1979.