I'm not certain if there is a low acceptable limit for fluoride since all the "science" around safe amounts is corrupted by the aluminum industry who wants to add their waste product to the drinking supply. Arsenic is naturally occurring in some drinking water supplies too, so just because it's naturally occurring doesn't mean its necessarily safe. If there was a dentistry benefit to fluoride it would be through topical application of it to your teeth, like with toothpaste, not from drinking it. Even then I use fluoride free toothpaste since the toothpaste tube says it's harmful if swallowed. I try to avoid it as much as possible, but I can't say there isn't a low acceptable amount.
I don't know if commercial water bottlers use a 4-stage RO process, or a 3-stage. I have a 4-stage reverse osmosis unit one that installs under my kitchen sink. They cost about $200-$300, so they payoff pretty fast if you'd otherwise be spending a few dollars a day buying bottled water.
Kelp is a very good source of iodine, the main problem with it is there is lots of mercury pollutants in the ocean, so you are potentially exposing yourself to that. You'd want to ensure it was tested for mercury. Nascent iodine is good, but expensive, Lugol's iodine is cheaper, but it is difficult to take small doses of. I've used both with good results.
