I'm glad to have helped. I am currently planning my own trip to Arran and Coigach at Easter so I promise I didn't have to think too hard :-)
The main, 'A' roads are fairly fast these days. Inverness to Ullapool is half an hour in normal conditions. The only problem is the 'high' passes at Drumochter, Slocht and under Ben Wyvis on the Ullapool road. These can be blocked by heavy snowfall, but they usually shut the snow gates in good time. I would recommend a basic winter car kit so you can sit out a night with a blanket and thermos, but you won't have to contend with really low temperatures.
The small single-track roads are often easier to drive at night becuase you can see oncoming headlights a lot further off than a car in daylight. On the other hand, the sheep always sleep with their backsides out on the tarmac, so you need to drive carefully all the same.