A jury was quickly selected yesterday in the trial of Rep. Daniel J. Flood (D-Pa.), the powerful Appropriations subcommittee chairman who is charged with trading his influence for money and bank stock.

The indictment which contains some of the most sweeping allegations ever brought against a sitting member of Congress, charges that Flood and a former aide, Stephen B. Elko, plotted to "corruptly influence" nine federal agancies on behalf of businessmen who paid them for their efforts.

U.S. District Court Judge Oliver Gasch estimated that the bribery, conspiracy and perjury trial would last about four weeks, with as many as 85 witnesses expected to testify.

Elko is among the government's chief witnesses and defense lawyers indicated during prelimainary arguments that Elko will be their major target.

Defense lawyer Walter Fleischer suggested in court that Elko had kept thousands of dollars in bribe money, transferring some of it to his girl-friend rather than paying it to Flood, as the indictment alleges.

Elko was Flood's administrative assistant from 1970 to 1976. He is now serving a prison term on a bribery conviction growing out of one of the deals mentioned in the Flood indictment.

The former aide began cooperating with federal prosecutors in late 1977 in a West Coast investigation which also produced the perjury charges against Flood.

The Flood investigation began in 1975, when a Senate subcommittee began looking into reports of payoffs by trade school officials on the West Coast.

That investigation produced the charge that two vusinessmen, William Fred Peters and Deryl Fleming, paid Flood $6,000 for help in influencing the U.S. Office of Education to accredit fheir West Coast Trade Schools so they would be eligible for federal funds.

Those named in giving money to Flood included Dr. Murdock Head a prominent Washington physician who heads the nonprofit Virginia-based Airlie Foundation. Head allegedly gave $27,000 to assure that Airlie would get federal contracts that were awarded without bids.

The jury was selected with relative ease from about 180 prospective jurors.

Flood slipped away during a lunch break to be sworn infor his 16th term in the House of Representatives. Ranking sixth in seniority, Flood is chariman of the House subcommittee that appropriates money for the Labor, and Health, Education and Welfare departments, both of which he allegedly influenced on behalf of the businessmen.