It is easy as you describe.
No, if you are quick to swap valves, the cooling system does not need to be burped.
Fifteen minutes DIY on a cold engine.



PART NUMBER SEARCH RESULTS WITH PRICES
Vehicle 1989 Mercedes Benz 300TE Wagon
Part: ACC Mono Valve
Note: Heater Valve
With heating water feed in top of heater core. Electromagnetic heater valve.
R3022-11894 ACC Mono Valve IN STOCK $146.02

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1YE1FF2PQ1YH00MHTN&year=1989&make=MB&model=300-TE-001&category=R&part=ACC+Mono+Valve