Thanks everyone for the imput. The primary reason for replacing the crank is the middle chainring is worn and must be replaced, along with the chain. I realize I can just replace just the chain ring but replacing the entire crank gives me the opportunity to go to a longer crank arm.
The reaon I am thinking about goint to a single speed crank is: 1) the derailer needs replacing but I do not want to sink a lot of money into an inexpensive bike , 2) I ride mostly level ground and have little need to use the high or low gear.