Mechanic advice needed?

just got head gasket replaced as well as valves but am now experiencing heater core problems? I got some kind of fluid coming from the dash above cd player. i believe it is antifreeze. It has come out of the vents but seems to not really have a color but is a little oily. my car was over heating when i come to a stop or let it it run without moving. Mechanic told me to have it flushed to remove any left over milky substance. He said i might need a radiator but to run the car then to get a flush about a week . i dont run the car really that much due to the heating problem and im waiting to have it flushed at radiator garage after the holiday. I just dont want to put any more money into the car but at the same time i cant afford to go out and buy a new one. Any ideas on what might the problem be ? or about how much it would be to fix this problem? its a mercedes so i know the price is going to be regardless but maybe if its a hose that i would not have to get into dash? would it be a real hard Do it yourself? i dont know alot about cars but im sure i could get some kind of repair guide and some friends help. any help or thought would be greatly appreciated.