the solution is better than nothing, but one point of ph equals a ten times change in concentration, so a meter is always a good idea. the difference between 5.5 and 5.8 is a LOT more than it sounds like.
a ppm/ec meter is a useful tool. its not absolutely necessary, but it makes it a lot easier to know exactly what you're putting in your pots, and to keep records, which makes it easy to grow the same strain again.
18 inches ought to be enough for a hid, may still be a little close for a strong led.
i don't have an opinion on the soil, i've been growing in coco and pro-mix so long i've forgotten how to grow in soil.
it does seem like your medium may be hot, maybe get some pro-mix, mix it 50-50 with your medium to start plants in,then put them in straight soil when they get big enough to repot.