I've replaced a steering lock mechanism when the key would NOT turn in under 3 hrs DIY (had to grind out retaining button on column lock), so I think you'll do much better than I did, since your key will release all components. Access thru the IP opening just as you planned and remove the knee panel as well. Loosen the steering column bolts (two) at the IP opening to drop the column a couple of inches to make it easier. Be very firm with the retaining "button" on the steering lock where it enters the column collar. It's a lot of push and twist work, but fairly intuitive.
The Golden Rule
1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)