There's manual bypass kit available for auto climate control, costs less than $100 from nuts to soup. You will lose AC control but gain heat - just installed the kit on my 116. Brake servo will be much more expensive repair. And 1979 300D is NOT the most desirable of 123 series. Later turbo models are more highly prized and so is the 4-spd 240D.

Meanwhile Winter months are buying season for diesels with millions of bargains. $3000 is high dollar figure even in Springtime. Do you need a car immediately?? If not then I'd spend couple months looking. How about 300TD square-window station wagon? They can be found at/near same price, considered highly desirable Classic MB's. Otherwise for this kinda money gimme a 108 sedan.