Brian,
I'll be a contrarian here - when I owned my 150 I regularly flew it into IMC, however, never ice or thunderstorms. I made one trip over a two day period of well over 500 nm to attend a conference, about half of the trip was in IMC. Nevertheless, the challenge is to have a solid alternate within the range of the airplane. That is sometimes not possible, especially with widespread fog.
If you are going to be carrying that much occupant weight, you cannot carry full fuel, I usually figure only about 320 pounds in the cabin with full fuel, especially if IFR because the rate of climb really drops off if you are over gross, and that matter if you are going IFR and have to get to deal with minimum enroute altitude.
For the distances and weights you are considering, as much as I don't like to admit it, a 172 would work better.
The 172 is only about 10 knots faster than the 150 in real life, but the endurance makes a difference when dealing with being able to plan for an alternate when IFR.
Don't rule out the 150 for actual IFR - I've shot a bunch of approaches to minimums in them, they do just fine. They are easier in turbulence than, for example, the Grumman Cheetah or Tiger, but they still require effort. Plus, when you break out at minimums and there is an airliner holding short, it's kind of fun to realize that the 150 is every bit as capable as the blowtorch when it comes to shooting that approach.
Best regards,
Rick
