A discussion under "Are Sermons Scriptural?" in MennoDiscuss:
a. Is there a NT basis for a single teaching elder in a church?
b. Were NT sermons extemporaneous or planned?
Of course, my opinions follow:
a. No. There is a basis for elders of a church-- whether one or more than one-- being able to teach (I Tim. 3:2). This might imply that the lion's share of teaching would fall on them, but not exclusively them. However, it is clear that within the church there were many different giftings and many people using those giftings in the church and many people had the same giftings and they all used them in public (I Cor. 14:26ff).
However, in order to prove that a church shouldn't have one pastor who does all the teaching, it should be asked if there is anywhere in scripture where this is not right. The only place I have is I Cor 14 again, which seems to indicate that everyone participates in the service who has something to say, but I don't know if this is proscriptive or descriptive. In general, I think that there isn't anything wrong with the practice, as long as the gifts of each individual are being used in the church.
b. They were both. Most sermons we have in the NT are extemporaneous, such as almost every example in Acts. However, Jesus had a set group of teachings that he presented in different ways in different occasions, but they were pretty standard. We have pretty much the same outline of teaching in Matt 5-7 and in Luke 6, in clearly different locations (unless you think that Matt and Luke invented the locations). Yes, there are differences, but the similarities are striking. Of course, Jesus also preached extemporaneously, but this wasn't his typical teaching style. When it says in Mark or Matthew "he taught" without any description of the teaching, we might assume that the teaching he gave in that location is presented elsewhere. It also says "he taught in parables in every place" in connection with the set of parables we have in Mark 4, which is almost the same as the expanded set in matt 13-- this set of parables he probably taught more than once.