Yes do EVERYTHING. Workiing Saturdays only, I started in January and just completed this job in June if that gives you some idea.
You'll want to remove the seats and center console. Invest in one of those Pilates pads for about $20. You can cut it in half and it makes a fabulous pad for your back and head while you spend all that time lying in there. Take lots of photos as you go in. When you think you have every possible angle, shoot a few more. I took a photo of each wire connection before I disco'd them and I put tape labels on each wire with a description of where it attached. I still had one that perplexed me for a bit while re-assembling.
I had the heater core replaced by a radiator shop that sent it to Ohio. It was nice work when it finally came back. That cost $225. The Mercedes price for a new heater core is $1,800.00.
When you get the defroster motor out, check the axle for end play. If it moves back and forth at all, get a new motor. Be very careful of the fan blades. You cannot buy them, you can only buy a complete blower assembly. I checked with the Classic Center; $2,400.00 I kid you not. Bud's Benz sells a perfect replacement motor for $150.
I put in the SPAL blower for the A/C. replaced the heater control levers, luricated the pull cables, replaced all the foam on all the vent doors and seals, the radio speaker, the dash wood trim, and installed a power antenna.
Good luck and I reommend aspirin fo the muscles and neck pain.
