Did Neville Chamberlain actually believe that he had achieved peace in Europe or was he simply trying to buy time for Britain to prepare for war?

The UK government does not depend on the Prime Minister, the Prime Minister has a cabinet of ministers. Neville Chamberlain was definitely in favour of appeasement but he had a cabinet behind him and he represented a continuation of existing British policy. There were also concerns about the economy and the state of the military. The cabinet had approved a large increase in investment in rearmament and on his way home from Munich, Chamberlain said to Viscount Halifax (the then Foreign Secretary): "We must hope for the best but prepare for the worst".

This is interesting because it is one of the themes of Robert Harris's novel Munich. He does like to write against a background of history though. This is an interesting comment from Harris when interviewed by NPR:

The best witness for my view, which I admit challenges the popular conception, the best witness is Adolf Hitler, and one of the reasons I wanted to write the novel was I came across a diary written by Joachim Fest, the German historian, who ghost-wrote Albert Speer's memoirs — Speer was Hitler's armaments minister. And he asked Speer about Munich, and Speer said Hitler was in a foul mood for weeks after Munich, and at a dinner party it all came pouring out. He said, the German people have been duped, and by Chamberlain of all people. And even at the end of his life in 1945, Hitler was saying, "We should have gone to war in 1938, September 1938 would have been the perfect time." And I think if the British and the French had gone to war in September 1938, Hitler might well have survived a lot longer and been much more triumphant.

It is a same that Chamberlain died (1940) before having a chance to write his memoirs so we don't know exactly what was in his mind. At no point did he claim in retrospective that the British needed to delay the onset of War, just that it was better to look for peace.

Source: Neville Chamberlain, Appeasement, and the British Road to War by Frank McDonough, pub Manchester University Press 1998.