Egyptian officials believe that President Anwar Sadat would not have agreed to his Christmas Day meeting with Israeli Prime Minister Menahem Begin unless he was confident that the talks would produce positive results.

Because of this value sense of optimism, Egyptians were cautiousin reacting to preliminary reports of Begin's peace proposals, even though most officials regard the Israeli leader's ideas in their present form as little more than a starting point for negotiations.

Begin's ideas contained "elements of change that could put in motion a process of growth," an Egyptian diplomat said. But most officials felt that unless he plans to go a great deal further in Sunday's talks at Ismailia, it was very premature for Begin to talk of jointly proclaiming peace to the world at a meeting with Sadat and President Carter in Washington.

The announcement today of the Ismailia meeting overshadowed developments at the Cairo preparatory peace conference, which resumed following a three-day break but recessed again after less than an hour.

After today's brief session, spokesmen for the Israeli and Egyptian delegations held a joint press conference to announce that the next meeting would be held on Wednesday.

The official reason for taking another day off Tuesday was that the Israelis had accepted an invitation to visit Sadat's native village. But, as the Egyptian spokesman, Morsi Saadeddin admitted, "Everybody is now in anticipation of the prime minister of Israel."

The meeting of Sadat and Begin is expected to be held on a long narrow island called the "Island of Horsemen," which lies just offshore Ismailia in Lake Timasah

Begin is expected to arrive at the nearby Abu Sueir military airfield - the same one from which Sadat departed last month on his trip to Israel - and will probably be taken to the meeting site in a motorcade.

Only one road leads to the island, which is linked to Ismailia by a 23-foot-long bridge, and it has already been blocked off by security forces.

Sadat has a rest house on the island, and he flew there today after a meeting with U.S. Ambassador Hermann F. Elits, who has been acting as a conduit for messages from Sadat to Carter.

Egyptian officials said that at the conclusion of the Sadat-Begin talks, the two leaders would hold a joint news conference which would be carried live on Egyptian television - responded cautiously to the few details of Begin's peace proposals that he revealed in his television appearance in the United States yesterday.

The Egyptians expressed, gratification that Begin has come so far from the absolutist positions he was taking after his accession to power early this year. But they said his ideas, especially about the occupied West Bank of the Jordan, form little more than a basis for negotiations - certainly not the last word.

One Egyptian expert pointed out several reasons why Cairo is less than satisfied with what Begin has said publicly. The Israeli prime minister while talking of autonomy for the Palestinian Arabs, was vague on the question of which state would have sovereignty over the West Bank and the Gaza Strip.

The initial reaction here was to agree with the information minister of Jordan, Adnan Abu Odeh, who said Begin's plan appeared to be an attempt to "regulate and organize, rather than end" Israel's military occupation of the Arab territories seized in the 1967 war.

In addition, the Egyptian source pointed out that Begin said nothing about allowing Palestians who formerly lived on the West Bank to go back.

"More than the question of what passport they will carry if they want to leave there," he said, there is the question of whether anyone will be allowed to return."

Nor did Begin reveal his intentions about the Golan Heights, from which Israeli withdrawal is a minimum condition for peace with Syria.

On the other hand, by stressing what Israel would be willing to do on the West Bank rather than the Sinai, Begin provided no ammunition for Arab critics of Egypt who say Sadat is acting only for himself, not for a comprehensive solution.

In any event, the Cairo conference is in effect on the shelf until after Sadat and Begin meet. After that it is expected to resume efforts, probably at the foreign minister level, to work out implementation of any agreements reached by the two leaders.

A subcommittee of legal experts continued to work on some technical language, and informal contacts are continuing among leaders of the Israeli, Egyptian and American delegations, conference sources said.