My W211 CDI has the PTC heater as well. 200A alt and it idles up to maintain charging voltage, pretty cool. Almost instant warm air. I didn't think my 603 took too long to warm up enough to have heat. Anything is doable with enough effort. 150A alternator on a 602/3/6 would be enough to keep voltage up for that time. Install heater grid on heater core. You would need some type of arduino controller to monitor voltage, engine coolant temp, outside air temp, heater discharge temp, and interface with the CCU so you use the PTC only as much as needed. I think a factory webasto would integrate much easier. Plugging the block heater in would be the easiest.

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83 300CD- sanden, dual p/f condensers, 160a alternator, ect
91 300TD- 722.6, #22 head, 3.5L IP, w140 manifolds, ect