The phrase: "Let us be sure we have considered everything carefully and have the appropriate resources" best uses formal language conventions.
"Let's" was replaced by "let us" to remove the contraction, which is a typical characteristic of common speech.
To "make sure" is also a verb which is used in every-day speech but is actually a derivative of to ensure. Replacing it by "be sure" is a way to make it more official.
"Organized" is a vague term, which you would replace by more precise phrases if you were being formal: it involves "consider[ing] everything carefully" and "hav[ing] the appropriate resources."
