It sounds as though all buildings have electric outlets served by the same transformer. If that's true you might try some of the wireless intercoms that plug in to an outlet & communicate over the power wiring. That's probably the simplest solution but it won"t be perfect. Expect to hear some constant hum & other noise when electrical devices turn on or off. Be sure to buy the units some place where they can be easily returned if not satisfactory. You might need to try different outlets in each building to find the ones that work the best. One advantage of these intercoms is that the steel siding has no effect.
Another option might be CB radios in each building with mobile antennas mounted in a good location inside the buildings. At that close range the radios would probably work OK through the steel siding and you would not likely get much interference from the rest of the world.