94 Plymouth voyaguer "water pump?

It seems the water pump is going on my van, its a 3.3 v6 engine and i've gottin a 1/2 dozen estimates and everyone is eithier hour and half to do the job to 6 hrs and of course cost from 250 to 800 bucks. Anyone know anything about water pumps. Does the timing belt have to be changed. Why the difference in estimates? How do you get someone who knows what they are doing and not scammin me.
thx