Thanks everyone for the input.  

I plan on a fully loaded test run this spring with the SR 18.5" vs my MN II.    

The long length of the MN IV seems like it would catch a lot of wind making both Stern and bow paddle on one side in a cross wind.  The MN III doesn't seem to have anymore room than my MNII, just a 3rd seat.  So I'm thinking SR 18.5" right now as it looks to have more room than the MN II.

You may want to also consider a Bell Seliga...I have run trips out of it with similar weights involved and was happy with it's performance, and the stability is terrific, easy on the portages too

I don't know where your from, but Spring Creek Outfitters has most of the canoes your thinking about and could test paddle them as early as late Feb. That's when I paddled one as a local lake there is open early as the town power plant keeps the water a little warmer. Spring Creek is near Virginia, Minnesota. The Mn iv is a big boat, just to license it is like triple in Minnesota. I agree with the Souris River 18.5. Check 'em out at springcreek.com

Of the possibilities listed, I'm only familiar with the Souris River Quetico 18.5' and 17'.  A Quetico 18.5' has lots of room, and should suffice for your journeying.  However, if you love the MNII, you might prefer the MNIV.  The loading should be done depending on wind conditions to get the stern or bow slightly heavier (lower in the water) towards the direction the wind is blowing from.  I like to bring a large day pack that weighs about 30 punds, that can be stowed behind the stern paddler or moved to the bow when paddling in a headwind.

I agree with the SRQ 18.5' recommendation. I used to own one and it will easily handle your load. It will also prove to be very stable which is important when kids, and their unpredictable moves, are onboard.

I think you'd do well with a Souris River Quetico 18.5' -- I'd envision the kids sitting in the center on one seat (you'd have to rig that up somehow) or possibly the 9yr old in a seat & the 5 yr old sitting on packs.  I've considered a MN IV but never used one -- I suspect it is a lot faster than a SR Q 18.5, but also harder to turn and a bit less stable.

I've always rented and have used SR Q18.5's twice & they're pretty light but very stable (even for fishing) & can haul a lot of gear.

For comparison, I've used SR Q17's also & they're noticably faster than Q18.5's -- The Q17 probably much more like your MNII in how it cuts through the water.

I'm sure others with more expreience than I will chime in also.

Best of luck.  -Tom

Planning a trip out of Pickerel in July with my Wife, 9 yr old daughter and 5 yr old son.  Maybe the B-Chain circle.  I have a MNII and have done trips with my Wife and daughter in it with enough room for everyone.  Now it is time for my son’s first trip and I'm concerned about having enough room to safely and comfortably canoe.  We bring three  40 to 50 pound packs, day pack, and the four of us total about 500lbs.
I know of the MN III, IV and Champlain, and Souris River 18.5' as big load canoes.  The MN IV may be the obvious choice but I would like to get some input.