I think wires will take a lot of work and a lot of cable!
300m though walls is not easy. I have never used a type of rf module with that kind of range for Arduino, but I have read about them.
Does each house have WiFi and internet connection?
A 2-digit 7-segment display may be easier to read than 25 individual leds. Is it likely that two slaves will be activated at the same time? If so, the display could flash their numbers in sequence. Also 16x2 lcd displays are very cheap.
I have to say, this project is not a good project for a beginner!