I don't think you have to go any deeper to replace the evaporator than the heater core; they are exactly the same job. And it's a big job, most folks here report ~20 hours of labor.

The evap used to be $200; I would expect it's gone up in the few years since I last looked at the price. Get one with a pre-installed expansion valve; you need to change it anyways and this will save you some time/trouble/effort.

Replace every o-ring in the system while you have it open. Brittle old o-rings are a common source of leaks.