Have actually done this. I didnt use special mortar but it makes sense if you want to. If the old fireplace didnt support the chimney breast then just brick up the hole. It will be covered with plaster and I bet the old brickwork isnt too pretty anyway. Put a vent in the middle to stop condensation and cap the chimney. If you are going to install a fire later, dont brick up the back just yet- you need to install a flue liner that goes inside your old chimney.
(For f. sake cover everything as that soot gets everywhere!) You will also need to ensure you have an air brick somewhere-your new fire wont gas you but it could burn all the oxygen in the room and suffocate you. Hope that helps