What follows is my opinion about what has been proposed to help your cold start blues:

Series or Parallel glow plugs are less important than having ALL glow plugs function. If they all work, that's good. If one fails in a series system, they all fail, and if one fails in a parallel system, you have 4 left. Slight advantage to parallel. I'll bet parallel requires a change of relay.

Valve adjustment must be correct to get maximum compression. Diesel engines are compression-ignition, you need it all.

You didn't mention any temperature problem, why is he replacing the thermostat? It won't affect starting one way or the other.

A block heater can improve your cold starts.

Blowby is a sign of leaking compression. It may affect cold starting, but not much you can do about it.

M-B never made a gasoline primer for automobile use, and I'd never consider installing one. I would be very leery of gasoline under the hood in who-know-what kind of container, sitting on a hot diesel engine.

If it doesn't start with good glow plugs, a block heater, a good battery and starter motor, it's time to rebuild or replace it.

Best Regards,
Jim