I think its easy to scare yourself with what might happen. A total comms / navaid failure requires an alternator failure, then fully discharging the battery. Not all that likely. If you have an alternator failure you're going toward VFR as fast as you can with as much as you can turned off. You'll be really surprised at how long the aeroplane will run on the battery. Unless you have pitot heat and a bunch of lights on, you should get a number of hours running on minimum electrics.
The 5 top most likely failures are:
1. Vacuum pump
2. Vacuum pump
3. Vacuum pump
4. Vacuum pump
5. Vacuum pump
After that its likely to be an individual component rather than a system, eg the AI.
My view is that a vac pump / AI / alternator failure etc is most likely to occur while you are flying on autopilot. So, its important to know where it is getting its information and how various failures will affect it. A vac pump failure on autopilot while you are flying along happily doing the quiz in the latest FSA magazine is an insidious thing.
I doubt that there is anything in ERSA that will help you. Especially since its essentially a VFR document, not an IFR document (and if you are serious about IFR, you'll most likely end up on Jepps). What you'll need is a deep & thorough understanding of the aeroplane's systems, good situational awareness and a good brain.
Is you autopilot rate based or position based? Analogue or digital? Does it take information from the AI or turn co-ordinator? How old is the vac pump? What vac pump system is there? when did you last test it? How old is the battery? Can you switch the alternator & battery separately? What do you do if there is a voltage regulator failure? Where is the voltage regulator? If its a twin does it have two voltage regulators or one for both engines (C310)? If you have 2 AI's are they both vacuum? Where are the static drains / alt static ports? Does the ammeter normally read zero in flight or show a charge? Do you have an instrument that shows battery voltage (eg JPI or some GPS units b& some digital clocks).
Forget ERSA, read the flight manual.