Bidirectional buffer

You can use Z for either, since HDL simply describes how your system must behave.

Z means that signal is not driven, just like a tristate output.

How the described function is actually synthesized into the device depends from the synthesis tool and on device cababilites: i.e. if the real device has tristate buffers available, a true tristate bus can be generated; otherwise the same behaviour could be implemented with logic gates if the bidir bus is internal to the device and other bus drivers are known.