If the battery is 12 V at rest then all the fuses should be 12 V as well. You have corroded fuses and/or bad earths and/or other electrical problems
The starter circuit is pretty simple: the key pulls in the solenoid which powers up the starter. If the starter is turning over slowly you have either a low voltage at the starter or a mechanical problem with the starter
As I said some time ago, wipers running, lights flashing & etc are typical signs of low voltage when starting a W124
There are very few parts to test to find the problem. It doesn't lie in the fusebox, in the central locking pump or in the N10 relay, it lies somewhere in the starter circuit or battery
If there's a current drain somewhere that's flattening the battery, or it has a bad cell, or both, you won't be able to diagnose the problem properly. So I'd start by testing the battery
Nick Froome