Unless your ready to jump in with both feet and have some experience doing this type of work, I would condider passing on this one. First things first you are going to run into more problems trying to replace the base only. The joint between the exsisting wall will never be right and prone to leakage.
To do this job right you need to gut the whole shower back to the studs. The shower base you have apparently a mudset base with either a lead pan or vinyl membrane. It must be completely removed. Depending on the condition of the subfloor it might have to be replaced, as well as floor joist repair, etc. As you can see you could open pandora's box. I tore out a tile shower on Monday, started out with a small leak which rotted the subfloor, joists, sill and exterior walls. Think about getting in a bath professional in your area for a look at the job or some advice.