Powerline might work, but there are always a lot of "if's" with these devices, one being that they tend to work very well when both adapters are on the same electrical circuit and not all that well when one adapter is on one circuit and another on a different circuit. A lot will depend on how the electrician wired the house and the circuit box, how much distance there will be to cover, and whether there are any high current devices like an air conditioner, refrigerator, etc.....something with a compressor that will be cycling on and off and possibly causing some disruption on the AC powerlines in the process.
If you want to try powerline, it would be best to set up a "test" first to see how they will work. So, you will have one adapter near the main wireless router connecting with a short ethernet cable. Position the other powerline adapter in the remote location where you want to install an AirPort to provide more wireless coverage. Connect a laptop or other computer to the remote powerline adaptor, turn off the wireless on the computer, and see if it will get a good, solid Internet connection over a period of several hours.
Post back with your results. If the connection works well, then the chances that the AirPort will as well are reasonably good. The AirPort will need to be reconfigured to work this way, and the setup is different than extending using wireless, but the setup wizard will take care of all of the settings for you, so you don't have to worry about what to change.