Coming from military aircraft, and having flown CAT II ILS down to mins, there is a big difference when flying a fully capable IFR airplane vs flying any GA airplane (and sometimes I get the FAR's vs USAF regs mixed up, so I'll try not to quote FARs). One thing that you nailed on the head is icing. I live in New England, and in the winter time, even 500' below the clouds, ice can accumulate. If I think there's going to be icing, I avoid flying IFR.
As far as take-off minimums go, I saw a post earlier that someone is ok with taking off at 0-0, or 50 RVR. Personally, I would not even come close to that in a GA airplane (we aren't even allowed to do that in the Air Force, mission and airframe depending). My personal takeoff minimums in a GA airplane is somewhere between 1000' - 1500'. This would allow me to takeoff, and if I had a problem I could stay VFR and pull closed to land. With that being said, it would be situation dependent. If it was a spring morning haze level, I would takeoff in less than that, because I know I'll be VFR at pattern alt.
For takeoff, I would NEVER takeoff below minimums for the airport I'm departing. If I really had to get out of there, if I had a problem, I'd at least like to get in the radar pattern and land on backup instrumentation. An option if you're really in a bind could be to takeoff below mins, with a sufficient departure alternate (if there was an airport within the same radar terminal area reporting better weather). You really have to do your homework if you're going to do that.
For landing, it's going to depend on your experience and comfort level. Maybe if you're new to the IFR would, set a personal minimum to 500' above lowest compatible approach minimums, and 2 miles of visibility. As you feel more comfortable, you can lower that down to 300' above minimums and 1.5 miles, or 200' above mins, etc. Having gone missed on a CAT II ILS, and seeing 75' on the radar altimeter without seeing the ground creates a pretty significant pucker-factor
Happy flying!