marc,
run a search, this has been disussed many times.
In short, the US have had more than 10 years of NVG civvie ops and what they came out with (in relation to your question) is simple:
Cockpit must be modified to DCO 275.
Single pilot ops are OK if you only operate to established and lit helipads.
if you operate to non lit, or non permanent helipads you must carry a second crewmember, trained on NVG and positioned on the other side of the aircraft in a position to scan the area the pilot cannot see. Ie back or front is fine, as long as it is on opposite side.
In all countries starting up NVG, weather is always contentious. NVG in a VFR aircraft or crew should be limited to NVFR minima. If that requires twin or IFR in your country, then so be it.
If you have an IFR aircraft and crew, then IFR weater is your min, but when on NVG you must have in flight VFR at your selected altitude.
I would suggest that the limitation should revolve around in-flight Viz, which is determined by both atmospherics AND illumination levels, ie say house sized obstacles must be clearly visible (including detail) at not less than 5000m to continue on NVG.
Contrary to Crab's statement above, and as you have flown them yourself, I would suggest that wx is easier to see on NVG than unaided (on ANVIS 6 or 9 omnibus IV), thus easier to avoid.
It is crucial to remember in all this, that the flight profiles are not military. In Oz, we are puishing to fly at 500, do a recce and land or winch. We are not trying to fly below 500, NOE, do tac approaches, evasive manouevring, nor formation. Any of these elements requires much more training and caution, and limitation setting.
Try Mike Atwood of Aviation Services Unlimited in the USA who is one of only two fully FAA certified NVG schools in the US. Mike will have all the info you require, is a very competent instructor, and has all the knowledge of civvy NVG ops you could hope for. Incidently, he is speaking at HeliPacific at Coolum on the Queensland coast 13th of july 2005 on everyting you have asked. Should coincide with tri nations time too!!
Also try Graham Gale, Chief Pilot of Otago Helicopters NZ who has just introduced the first civvy NVG ops into EMS in New Zealand using the FAA template.