To always get latest plugin take for example my plugin:
http://wordpress.org/extend/plugins/wordpress-file-monitor-plus/
the download link for the latest is:
http://downloads.wordpress.org/plugin/wordpress-file-monitor-plus.1.1.zip
but if you remove the version from the download link you always get the latest version:
http://downloads.wordpress.org/plugin/wordpress-file-monitor-plus.zip
EDIT: Have you considered keeping a folder of the latest wordpress and plugins unpacked? Then as soon as a new plugin or wordpress comes out you simply unpack that over what you have. Then your bash script just packages the whole lot to be used on an install.