A 1984 300D.... ?
and there is another thread saying you are picking up a 1984 300D...
very suspicious....

Why do you think the AC is low on charge ?

With a low charge the AC should not even be coming on.
If it is coming on and then stops cooling then it can be the expansion valve failing to keep the evaporator above 32 degrees... if it gets below that then ice forms in the air flow path....making it feel like it is not cooling.

Have you checked that Flap which , if not closed, allows warm air to mix with the cold air, making it feel like the AC is not working ?

That ought to keep you busy a while....