This section is unfinished, and I'm not sure when I'll get around to doing so.
I haven't designed any custom printed circuit board for the stations, I've just used prototyping board (like veroboard). You can see a picture of them in the topless photo of the prototype base station.
If you do design a board, I wouldn't mind being sent a copy. Whether that's an empty board through the snail mail, or a file that I can print out and draw a board from. I don't use any CAD programs, so I probably can't use a specialist PCB file format.
You don't actually need to make a base station, it's just a handy junction box to connect all the power and audio wiring together. You can make all your stations stand-alone belt-pack units, and just have a box of sockets to join all the stations audio and DC power wiring together.
I built a mains power supply into the unit. But if you're not proficient in safe mains wiring, or doing so isn't legal in your location, buy a ready-made regulated 12 volt DC supply, and plug your stations into the low voltage side of it. All aspects of mains wiring needs to be mechanically strong, and electrically safe.
Each station only uses about 10mA of current, so multiply that by the number of stations, then add a bit extra as spare, to work out how big your power supply would need to be. e.g. Six stations would use 60mA, and a 100mA power supply would give you plenty of spare current, should you want to add any more stations, in the future. You can use power supplies capable of providing more current, such as a 1 amp supply, the intercom stations will only draw as much current as they need.
It's supplying the right voltage that's more critical. Much less than 12 volts, and it won't work very well (9 volts seems to work reasonably well). But if you go over 12 volts, you'll blow up the LM386 audio power amplifier IC, some variations of the IC have a 12 volt limit. Use a regulated power supply, not an unregulated plug pack (where the voltage can go dramatically higher than you'd expect, when it's not supplying a lot of current). Or, you'll need to put 12 volt regulators into each intercom station, and supply them with 15 to 20 volts (series/shunt regulators need a higher input voltage than what they output).
Some voltage regulators are noisy, and will make your intercom audio noisier. For instance a LM7812 regulator is noiser than a LM317 regularor, and switchmode regulators are even noiser, still.