Lindsay -
Sorry for the delayed response, I must have missed this somehow.
The Marian - Camsell route between Great Slave and Great Bear is a traditional route of long standing. There is a lot of good information on it at this site, published by the museum in Yellowknife:
http://www.idaatrail.ca/The section of the route from Behchoko (formerly Rae) and Gameti (on Rae Lakes)I paddled long ago (1981) and I don't have my log for this section in electronic form. In summary, it was about 225 km, and took 9 days of travel, 2 of which were shortened somewhat by wind issues. We made 29 portages, all short, on this section, and found good trails in most places. I think trails improved after that - as the school trips from Rae started using the river - but were then affected by the fires a couple of years ago. However, I know people who have followed the route successfully since the fires, and there are a couple of parties who were doing the route this summer, so more up to date information should be available shortly.
Basically, the Marian is a small river - particularly north of the Emile and LaMarte junctions - with frequent but short portages. Near Marian Lake the river flows through a marshy area; the terrain becomes rockier and the lakes larger as you head north.
After portaging from the west side of Mazenod Lake into Sarah Lake - the headwaters of the Camsell - it is all downhill to Great Bear. This section of the route is predominantly big lake paddling with only occasional portaging, and you can make very good time if the wind cooperates.
You could follow the Camsell all the way to Great Bear, but this is not the most direct or the traditional (Idaa) route. That route avoids the section of the river flowing out of Hardisty Lake to Isabella Lake, and goes from the west side of Hardisty through Lac Malfait and Beaverlodge Lake to reach Hottah Lake.
It is also possible (and perhaps quicker) to go directly from Rae Lakes to Taka Lake by portage, and then from Taka to Seguin by a portage route. Both of these options are probably more direct than following the river.
I think the total Great Slave to Great Bear route is likely just under 500 km, with around 40 portages, depending on the exact route chosen north of Gameti. It is a well established route and generally easy going, with the frequent short portages on the Marian and the possibility of wind on the Camsell's big lakes being the main issues. Three weeks should take you comfortably to Great Bear, and it could be done in less time if the wind cooperated.
I hope this gives you something to think about. If your plans become a bit more definite, let me know, and I can provide some more details and contact information for others who have traveled the route recently.
-jmc