Good to hear it worked out for you.   That first mechanic was totally
incompetent.  Most of us here who aren't even professional mechanics,
but work on our own cars, could have found those things very easily.
The hardest thing there was probably the battery drain.   Even that,
unless it's intermittent, can quickly be found to exist by just
connecting an ammeter to the battery cable.   Finding the actual
location of the problem causing it, can be more difficult..  But
clearly the first guy didn't even know how to tackle any of this.

We appreciate your letting us know what "the problem" actually was.
Appears you have found an able mechanic who will now take good care of
your 300SD.