You need to first decide on the GM tube you are going to use. The power supply comes next. I do not know what you are going to talk via RS-485.
The power supply can be a very simple boost circuit but the output is not regulated because regulation is not needed. As long as the output voltage stays within the GM plateau you should be fine.
The power supply should be matched with the GM tube you are going to use - not the other way.
Exactly what sort of information are you planning to transmit over RS485? Total pulse count? Repetition rate?
The GM tube provides pulses whose repetition rate is proportional to radiation....the simplest solution would be a digital line that outputs a pulse each time the GM tube does so.