Congrats!

In terms of the cost beneify ratio, taking supplements and/or electrolytes is an easy yes. Complications of deficiencies range from uncomfortable to downright scary. For a short 3-5 day or even 7 day fast, most people should be ok with just electrolytes.

So most people use table salt and "no-salt" (potassium chloride). 1 teaspoon each in 2 litres of water will give you roughly 2500mgs of sodium and 2500mgs potassium. Drinking at least that mixture everyday, and any fresh water you want, should keep you properly hydrated. 300mgs of magnesium citrate at night, and 3-400 mgs calcium citrate am and pm is a good idea too. I would also take a high quality multivatimin with zinc when you resume eating too to make sure the other vitamins and minerals are topped up.

Many people have done water only for extended fasts and appear to be fine. But given that supplements are an easy way to reduce the stress of fasting on the body, including the kidneys, and there is no upside to restricting minerals in the literature, I say go for it!

What I wrote out above is roughly what I do, except I use potassium bicarbonate instead of chloride, and supplement with selenium and iodine for thyroid support. Actually I also take copper, manganese, and chromium too when it's been over 3 days.