でよければ

でよければ offers something modestly: "if X will do", "if X is all right with you". It follows a noun – yourself, a thing, a time – and the second half says what you will do. It is でいい, "X will do", in the ば-form.

Offering yourself, わたくし私でよければ, is the commonest use: you may not be the best person for it, but you're willing.

でよければ or よかったら

よかったら and よければ without a noun mean "if you like" and leave the choice to the listener. でよければ names what you are offering and plays it down.

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In formal speech and in business, でよろしければ takes the place of でよければ. Without a noun, よろしければ is the polite "if you like".