President Reagan will accept Soviet President Leonid Brezhnev's invitation for a "well-prepared" summit meeting in a third country in October, according to informed administration sources.

The invitation has been discussed favorably with Anatoliy F. Dobrynin, the respected Soviet ambassador to the United States. The most likely third countries, according to an administration official, are Austria or Ireland.

The Brezhnev invitation, issued April 17, was a counteroffer to a proposal by Reagan for a meeting in New York in June. The Soviets viewed Reagan's statement as a political one designed to defuse growing concern among Americans about the nuclear arms race and demonstrate that the president is a man of peace.

The Soviet counterproposal also was viewed suspiciously in Washington because of its timing. It came two days after the 75-year-old Brezhnev was reported near death in a Moscow hospital and appeared designed to quash these rumors.

Nonetheless, the Soviets have long desired a summit, and the president is described by administration officials as now being ready to accept. He also is expected to convey this view to European leaders during his trip to Europe next month.

The White House and Sen. Bob Packwood (Ore.), chairman of the National Republican Senatorial Committee, are about to kiss and make up, at least for the duration of the 1982 election campaign. But it will be a marriage of convenience, not true love.

The committee destroyed 8 millon copies of a controversial fund-raising letter, produced at a cost of $2 million, after the White House objected to the prominent use of Packwood's name and photograph on the letter, written over a fascimile signature of Reagan. The Oregon senator's persistent sniping at the president--he said April 14 that the president's abandonment of a balanced budget goal had "removed the glue that held everyone together in the Republican Party"-- has made him the least favorite GOP senator at the White House.

But the retaliation against the Packwood letter has produced an adverse effect, reducing contributions to the Republican unity dinner that will be held for congressional candidates in Washington tomorrow and cutting support services to GOP senators running for reelection.

Last week, in a solution devised by the two Bakers, Senate Majority Leader Howard H. and White House chief of staff James A., both sides agreed that it was time for an armistice. The solution calls for Packwood to agree to refrain from further criticism of the president, while remaining free to vote his conscience on legislation. Reagan, in turn, will sign a fund-raising letter for the senatorial campaign and the White House will try to turn off an anti-Packwood campaign by conservative GOP brewer Joseph Coors, who sent out a letter of his own urging Republican contributors not to contribute to the Packwood committee.

"There were no winners in this fight, just some prospective Republican losers in November," said one administration insider last week after the compromise was worked out. "Neither the president nor Packwood have anything to gain by allowing this to escalate."

Reagan will take to the road again Sunday to do the commencement address at his alma mater, little Eureka College, near Peoria, Ill. He'll use the trip as a springboard for political campaigning in Chicago the following day. On May 14, the following Friday, the president will make a campaign visit to Philadelphia. Both trips reflect current political thinking at the White House, where retention of Republican governorships in Illinois and Pennsylvania in 1982 are viewed as a key to success in 1984, particularly with the voluntary retirements this year of GOP governors in Michigan, Wisconsin, Minnesota and Iowa and the constitutional prohibition of Ohio Gov. James A. Rhodes' seeking a third consecutive term.

There is less clarity on what Reagan should do politically outside Washington to advance his proposal for a "bipartisan compromise initiative" which some critics have described as a campaign against his original budget.

One plan calls for prompt deployment of presidential surrogates as part of an "action plan" to sell the budget. Reagan, the administration's best salesman, is likely to be making some of these trips.

A week ago the administration dodged another confrontation with Congress on the capabilities of Central Intelligence Director William J. Casey when the president decided to name veteran CIA bureaucrat John N. McMahon as Casey's deputy after the popular Bobby Ray Inman leaves the No. 2 post. But though Casey enjoys the confidence of Reagan, there are few illusions about his ability among top White House staffers. As one of them put it recently, "Bill's the only CIA director we've ever had who doesn't need a scrambler on his telephone."