Everyone who does their own rhinestoning has different opinions about what's best, but here's mine:
Most important, forget Korean stones! They do not show up on the floor at all. If you want to save money, go for Czech stones (i.e. Preciosa brand). Some people even think they have more shine from a distance than Swarovskis.
I use flatback stones and Gem-tac to affix them. Some people prefer E-6000 for glue; it seems to be sturdier, but is harder to work with. Both are available at hobby and craft shops. Some people put a few dots dots of glue at a time on the dress, then place the stones on the glue. I prefer to dip the stone in the glue. Either way, you need to use just enough glue that a little comes up around the edges of the stone for the most secure attachment. It will dry completely clear and invisible. People use tweezers, birthday candles, or a blob of beeswax on a toothpick to pick up and place the stones. Sounds harder than it is, once you get the hang of it. Hot fix stones really require a special tool and it takes about 30 seconds to warm up each stone. It would be very hard to use an iron and get the stones placed as precisely as you would want. I'd recommend practicing your stoning technique on a t-shirt or something before you start on your dresses.
SS20 and 16 are the most common sizes. Anything smaller is very hard to work with. How many depends on the dress and the stoning design you want. I usually buy a small amount of stones, lay them out on part of my pattern, count them, and then multiply to get an estimate of how many I'll need in total.
AB or not depends on the color and effect you want. AB coating tends to give off blue and pink highlights, on top of/combined with whatever color the stone is. It's worth buying a sample card, that has every color of stone on it, usually on a sheet of clear plastic, so you can hold it over your fabric to decide what colors are best.
My favorite vendor is rhinestoneguy.com.
I've never cleaned a dress, but I know some people handwash or pin them in a pillowcase and use the gentle cycle of the washing machine. You can replace any stones that come off. I don't think you can dry clean the glue.
Good luck!