How do the reasons Mr. Collins offers for wanting to marry Elizabeth indicate that he would have married any of the Bennet daughters?

His reason are those of convenience. It doesn't really matter which of the sisters he marries, in fact any of them will do. The same goes for the women...... if any of the girls marry Collins, the women in the family will have a sense and form of financial security. Mr Collins comes out ahead simply because he enters his ministry with a wife by his side.