Given your rigorous cooling system maintenance and use of MB (or Zerex G-05) antifreeze, a flush is not really necessary - just thorough draining, including the block. - open all the drain valves, let the coolant completely drain, close the valves and fill with a 50/50 mix of the correct antifreeze and distilled water. On my inline six both the block and radiator drain valves have nipples to attach drain hoses, so the job is easy and clean with no spills or mess. It's the easiest antifreeze change for any car I have ever owned.

I flush my system sometimes, but I'm not charging myself $80 an hour. If you're using a professional mechanic, the extra time/cost of flushing is probably not worth it if you change at MBs interval and use MB approved antifreeze. I have never removed the thermostat housing or changed the themostat. In fact, I have never changed a thermostat on any of my cars, and they range in age from 14 to 42 years (all bought new). Proper cooling system maintenance is good for the thermostat and there is no need to replace them unless they malfuntion, which is easy to diagnose.

Follow MBs recommended change interval listed in your owner's or maintenance manual. For my car it is three years.

Antifreeze corrosion inhibitor additives are depleted with time, whether the engine is operating or not, which is why it needs to be changed periodically, however, operating temperatures deplete the additives quicker, which is why some OEMs have a either/or time/mileage interval. In MBs case they just list a time interval, regardless of mileage - at least that's the case for my model. Given your low mileage accumulation, you would not exceed most manufacturers recommended mileage interval.

Be sure you also follow MBs brake fluid change interval, which is probably two years for your car (see owner's/maintenance booklets). This is especially important since you live in a damp climate. If you lived in Scottsdale, you could increase the brake fluid change interval to 3-4 years without consequence because of the dry, desert climate.

Duke