Heater and defroster issues

My boyfriend has a 1991 Toyota Tercel that he got from his sisters boyfriend who had apparently treated it kinda bad.. My concern now is the heater and defroster. We cant seem to keep the windows from fogging up at all and keeping the inside of the car warm is nearly impossible because the heater gives of very little heat and the heat that should come out of the vents directly under the windshield doesnt. An the heater itself makes a loud buzing noise. My question is what do we replace? Right now Im thinking that replacing the whole works would be best but any suggestions that could save us some money would be great.