Can I select any other field apart from those shown in the drop-down list in EmailMerge Std Version 1?

EmailMerge Pro version supports all the fields whereas EmailMerge Std version only supports the fields shown in the drop-down. You may still be able to use EmailMerge Std by copying the information from the field you want, to one of the fields supported by EmailMerge Std (if you are not already using that field).