which instrument manufacturers I should explore that offer a station assembly with POE data retrieval and a heated rain gauge for snow country
I am not sure what you are getting at here. Davis, for example offers the Weatherlink Live which can function over Ethernet or WiFi. The now-obsolete WLIP also uses Ethernet.
The WLL can be located indoors and doesn't need to be near the outdoor sensors. A heated rain gauge option requires 120VAC
to 24VAC power supply/transformer to power a large wire-wound resistor that forms the heating element. (I provide my own 24VAC power via a transformer)
I would question trying to run a rain gauge heater using POE. Only the latest spec POE devices even come close to delivering the amount of power needed for just the heater.
The Davis WLL (and all consoles and Envoys) run on 5V wall-warts and do not support POE.
In the past I was able to power a webcam via a POE adapter, so one of these might work to power a console, WLL or Envoy. As far as other brands, I just don't know. Maybe other owners of the various brands of stations will chime in if they know about one.
A small clarification on the power for the heater resistor. It is not run directly from 120VAC. Sorry for the way it came across. Greg H.