Assuming the engine is getting up to temperature (80C) you lack of heat is almost certainly due to the monovalve:

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ428Y1CDDBN&year=1982&make=MB&model=300-DT-001&category=All&part=Mono+Valve+Repair+Kit

The center vents are not intended to blow heat, they should be shut off in heat mode. The car failing to stop right away is just a vacuum leak (assuming your power brakes are working, it's not a vacuum pump). Take a look at the vacuum lines under the hood first.