Yes these things can work though you do not need to buy anything, well maybe new electrolyte.
Even batteries with less than 12volts can be recovered. I haven't seen any do as well as new ones on a load test but if one does not mind having marginal batteries then these methods are fine. Maybe carry a spare battery or two as they can be had very cheap.
I would, I have, just drained the old acid. Don't keep. New acid is cheap (was cheap) and clean and the best way to go.
Fill battery half way with distilled water, seal, and shake. Empty dirty water, repeat until water comes out clean. I'm sure salts or something would help but all the batteries I did this with had so much crap in them that shaking them would clean the plates well enough.
Then add acid, fully recharge, check voltage, load ect. If it didn't work, keep acid for next attempt. Rinse battery out well, break apart, melt out the lead. Makes for great kellets or weights.
In one case a H-D (of course) had a dead battery, and of course no money (none of us did in those days) and we were about 1/2 hour from nearest town (folks didn't like us sleeping in town, go figure). He had been having problems with the battery for a long time but this time a boost wasn't enough. I took the battery, poured the acid, more like water, out, filled with 7-UP (wasn't going to waste beer) shake, rinse, shake rinse. Filled up with acid from a car battery, gave bike a boost and it was on its way. A year later I see the same guy. He was real happy as he still had the same battery, so was I as I wasn't living in campsites anymore.
Of course there is so much that can go wrong, batteries can explode, particularly old batteries with problems, you can spill acid and not know it until things start to burn, you can splash acid and lose your eyesight, a stupid kid from next door can find the acid and think hey must be vodka lets drink it, lead is toxic and so on.