If you're replacing the timing belt anyway it only makes sense to install a new water pump, since 99% of the work to get to the pump is alread done getting to the belt. If you need a new water pump you'll have to remove the timing belt to get to it, and you may as well install a new belt when you put everything back together.
This is not a simple job. You'll need lots of tools, including a torque wrench, a good service manual, and a safe place to work on the car. You may need jack stands, too. How much experience do you have working on engines? I wouldn't want to tackle this as a first time project.
Lots of things have to be removed before you even get close to the timing belt and water pump. If you're considering doing this yourself, I suggest reading through the procedure in a serivce manual, and then deciding whether or you you think you're up for it.
I've replaced timing belts on several cars, but I've paid mechanics to do it on others. Some are more complicated and difficult than others.