The Nienhuis set is superb. And expensive. If you are purchasing the set of four boxes, which is an important staple in your home classroom, try sharing it with a few other familes. Four families can purchase it, each one takes a set home, and then the children can meet to work on the different sets and then swap the sets to take home (or take home their own set).
This exchange also serves to put children together in a group environment, which is a key part of the Montessori classroom as well.