Gorwingel is right as to why Christopher Lee did not play Gandalf. If you watch the
Cast Interviews on the
DVD Extended Edition of
The Fellowship of the Ring it is explained there.
The role of Gandalf was physically demanding. Riding horses, walking, running etc. It also had a greater time commitment. Christopher was realistic enough to realize it would have been too much for him.
I believe he would have made a fine Gandalf had he been younger. He looks (and sounds) great for his age!