I've done a 40 day water fast. You don't need any supplements if you drink enough water and slow enough that the electrolytes don't get flushed out.
But it would seem to me to be an unnecessary risk if the goal is to die by SN. Most report a relatively peaceful death as it is. Putting your body into a completely different state means other people's results are less reliable for you. Terminal cancer patients are often recommended to fast as a last resort because in the absence of processing food and fueling muscles, the body can focus entirely on healing. So while it is weaker and lighter, it's likely better equipped to fight the SN than it otherwise would be. You could increase the likelihood of death by not drinking in the last 48 hours or so, but that's going to drastically increase pain. And the fast doesn't put you into some bliss state where no pain exists. Occasionally you get little highs which allow you to think clearer than usual but it's not 24/7. I felt more or less the same as I do now most of the time, although others have reported it being painful at times too.
Paradoxical as it may sound, I'd only do a 40-day water fast in your situation if I wanted 40 days to contemplate life and how to proceed (or not). I'd try to minimise distractions and just ponder life and death. And if you ended up deciding SN is the way to go, I'd still likely wait until a week or so post-fast and follow the usual method so the chances of things going wrong are minimised. If you're certain SN is the way to go already, I think you'd be better off using the tried and tested methods here and skipping the fasting idea.
