The rear view mirror has to be pulled off; it is clamped tight by a spring.
Is it possible that the metallic box belongs to the cruise control (which is under the dash at the driver side as well).
You can order all kind of locks by chassis# at your local dealer and the euro prices are very reasonable.
All W124 share the same dashes, so it doesn't matter whether you can or cannot find a leather optioned donor. Even my 300CE24 conv has the same dash as the average 200D (that is used as a taxi...)
W124s can be found on every salvage yard in europe (where do you live...Italy, isn't it?) and finding under dash parts should be fairly simple.
The W124 interior is not that hard to work on...
good luck & greetingz,
with a minor 600+k