Another vote for Harveys. They are superb and the BMC Mountain map of the area is a triumph. It's light weight, fantastically clear and printed on a waterproof plastic stuff - so much better than the armour-plated O.S. laminated equivalents. The scale of map that you take will depend on the season in which you walk. I tend to use 1:25 000 in summer when the detail on the map is visible, and therefore usable, on the ground; in winter, when a good deal of that detail is masked by snow and drifts, the 1:40 000 (Harveys) or 1:50 000 (O.S.) are perfectly adequate. It's worth mentioning that the BMC are now offering a Silva Type 4 compass that has the 1:40 000 roamer scale on it, making it that bit easier to work out coordinates accurately.
To my mind, that whole area - the mountains, tops, valleys and Caledonian forest - is one of the most beautiful areas in the whole of the U.K.. You're sure to have a superb time up there. Enjoy!