Hi Ausnomad
The symptoms you describe suggest that you have a problem fairly high up in the electrical heirarchy, as it is affecting multiple items. With most vehicle electrics, the power starts at the battery and then divides down in stages to the individual items. There are fuses and divisions on the battery, then the engine bay fuse box and finally the under dashboard fuse box. Think of a tree trunk splitting into branches and then twigs !
You are looking for bad/corroded contacts (which includes plug-in style fuses and their holders), bad bolted joints etc. Don't discount the possibility that it's a problem with the ignition switch itself. Note that the electrical switch can be detached from the end of the lock barrel and replaced, you don't need a new lock and keys etc
Earth connections gather up all the return currents from the body/chassis and the engine/gearbox and take them back to battery negative. A fault with the engine/gearbox to body/chassis earth strap will affect anything that returns to the engine block, which includes alternator, starter motor and glow plugs. Usual symptoms are sluggish starter turnover or the battery not charging properly. Forget about checking any of these high current connections with an ordinary multimeter, they should have maximum resistances measured in thousandths of an ohm so different techniques are needed. Often cheaper to just replace anyway.
Diagnostic software including
MultiECUscan is a useful thing to have, and is cheap compared to garage labour. However, with the sort of problem you are experiencing I fear that you will just get a long long list of fault codes as multiple electrical items all bleat about not getting enough power.
Hope you find a good auto electrician. I attach a few schematics for earlier models, they won't be exactly right but might act as a guide.