Diagnosing 1978 300cd climate control questions

Hi I have a recently acquired 1978 300CD with a shot AC system. It has the automatic climate control system but only DEF blows and it doesn't even blow hot. The climate control buttons and control pad are soldered and look good. The servo on these models is apparently always the reason it goes bad. The previous owner disconnected the servo in fact, perhaps because it was draining the battery. I want to fix this sytem myself by getting new or refurbished parts and I know where to get them pretty cheaply, including a new aluminum body servo they make now. Questions: 1. How should I start diagnosing the fix for the climate control? 2. If I can isolate the servo as the only problem, how difficult is it for me to install a new one and recharge it myself with the proper refrigerant? 3. AC aside, why will the def not even blow heat and how do I diagnose the problem? It must be a hose or a heat valve not working eh? 4. If the ac servo is broken does that also prevent heat from going to interior? The blower works fine, the fuses are all new. I don't want the manual bypass kit. thanks!