Much can depend on how long it takes for the Tstat to open with the heater running. If the car was sitting and not run for two years, I would most likely drain and flush the system, start fresh. Even if you had proper MB coolant, it cannot be good for it to sit stagnant in the engine block or radiator for 24 months. I am no expert, but I did just replace my water pump and all the hoses etc and radiator. When I finally got the water pump off the block I had about 1 gallon of fluid that spashed to the ground (I raised the car in the rear when I did it so it would all fall out). I also think there is some small amount of evaporation that could occur. but in your case, I bet the heater core and the block are holding on to most of your coolant. The level, on level ground should be right even with the center seam of the coolant tank. Once you hit about 85C that Tstat should open up for you and let the fluid flow. When I refilled mine, I had the car raised up almost enough to let the tires off the ground, and I ran the car with the cap off, carefully squeezing the top hose to burp the air out of the system, and kept adding fluid after I got to temp. So I would fill with distilled water until the overlfow is level and run to temp and evaluate. Then I would flush the old and put in new Zerex or MB coolant/distilled water.