don't know how cold it is where you are, but...
I would bleed the entire system before I do anything. If the problem has gotten worse over time, it may not be air in the system, but it's the cheapest place to start. If the fluids have not been changed in 3 years, there is likely water in the system, and fluid which is less than optimal. Check for leaky calipers when you do (they seem rare on a MB).
If the fluid is old, seals are probably gone. From what you described, I'd swap the master cylinder (you could rebuild it, but if you have never done one, get help). A new mc will give you peace of mind. If the brakes don't feel nice and firm and pedal travel is still variable after that, probably time for new or rebuilt calipers as well.
1985 300TD 4-speed 212K
1992 400E 343K
2001 E320 72K