Before diving into your proposed undertaking, I’d suggest doing things in “stages”. First stage being getting the heater core issue fixed as it’s possible that it was “ground zero” for the blend door/actuator issue. Once the heater core is no longer leaking coolant into the duct work you can “wade” into the next stage of your project. I think that you will find it to be a MAJOR undertaking! Reading threads (out of curiosity) in various forums and watching a few YouTube videos (which might be a good suggestion) made me hope to never have issues with these components!
Once you gain access to the problematic blend door and its actuator, you may find that the door itself is the primary problem (they’re made of a fiberous material and disintegrate over time, especially if soaked with coolant!) The actuator/stepper motor may still be functional. Aftermarket (aluminum) doors are available. Install the new door, replace the actuator, only if necessary. At that point I’m guessing you’ll be wanting to “call it a day”!
