I've replaced several heater cores over the years... It's always "difficult" but a good torque adjustable electric screw driver is invaluable.
That and a compartmentalised tray for the screws and clips. Keeping them grouped and in order helps a lot.
I own a 1992 400SE - W140 and fingers crossed I never have to change that one! (The heater cores are notorious for failing)
This is my VW Golf VR6 from a few years ago. Awful job...
