Other belt you are thinking of is probably for power steering pump, its a very small v belt. The ribbed serpentine belt usually only drives the alternator and AC compressor on Toyota engines with a separate v belt for the power steering pump driven directly by the crank pulley.
You don't *have* to do a timing belt when you put in a water pump. It's just a good idea since it's a pain to get to all that stuff and you might as well do it all at the same time since it's all in the same place, and it's only a $30 belt.
Toyota water pumps are usually metal impellers, but the bearing life is about the same service interval as the timing belt and the only real concern is shaft slop and annoying knocking/ticking sounds as a result.
If you've got a lot of miles, say 100k+, might as do all the front seals also (front main crank seal, cam seal, oil pump seal) since they are right there. Usually you can get a timing cover kit that has all the seals, timing belt, timing cover gasket, tensioner, everything under the timing cover all in one set.
As for the labor? Having to pull all those brackets, mounts, pulleys, timing cover, etc, off with 1" of clearance to the inside of the fender isn't very fun, and you usually have to have it on a lift up and down numerous times to tackle the upper and lower portions separately. 10 lbs of shit in a 5 lb bag.