Well, since you asked! :p
1- I'd flush with citric acid and a forced-open t-stat per the MB factory procedure. Bad you for not flushing when you bought the car! Bad, bad!
2- Given the short time allowed, I'd at least use a McParts cooling system flush if you can't locate citric acid powder on short notice before your trip.
3- If the t-stat is not recent, replace it. DO NOT use an aftermarket no-name 'stat from McParts. Use OE/OEM Behr or Wahler. Trust me.
4- If the temp still is low, try the heater. If the heater blows nice & hot, the 'stat is probably FINE. In that case, you need a new temp sender in the head for $10. As they get older, they can read low. Had the same problem on my '84.
5- Use the amber/gold colored Mercedes, or Zerex G-05, anti-freeze only. No green or bright orange junk!
6- I assume you know not to stick a cold garden hose into a warm engine when flushing. Let it cool first - shut off engine, leave heater on MAX with fan on high, and the key turned on. The electric aux pump will push coolant through the heater core and cool things down in 10-20 minutes quite nicely.
7- I'm concerned about the coolant loss. If you can't find ANY external leaks anywhere, I'm afraid it may be a head gasket...?
HTH,