I recommend go with used intercoms available on eBay. A search for Radio Shack Intercom on eBay gives lots of FM type intercoms that work over mains, at least it does in the USA. Your visitor will need to know to push to talk in order to have a conversation with you.
The advantage of going with a simple wired intercom is the visitor does not need to push to talk. There is a call button that acts like a door bell, but other than that, the visitor does not need to do anything and you don’t require any skills from the visitor (could be important for some visitors). Whenever you turn it on, you’re listening to the visitor, and at the master when you push to talk the direction of communication switches to talk to the visitor. Disadvantage is of course you need to wire it. If going more distance than the wire that comes with the unit it may be a good idea to use heavier wire, such as coax or romex so you will not have much transmission loss.