If I am rebuilding the OCC heads, I like to use 24 gauge. If I am building on the RBA deck, I like to use 26 or 28 gauge, depending on which juice I am going to be using. For my thin mints recipe, it seems like 28 gauge works best. For my cinnamon roll mix, it seems like 26 gauge works best. It all comes down to personal preference. As cheap as kanthal is, I would just get several different gauges and try them all out and see what you prefer. A 100 foot roll of kanthal can be had for about 6 or 7 bucks, so getting a few gauges shouldn't break the bank.
As for batteries, get yourself some batteries that are rated for at least a 20A CONTINUOUS discharge. Don't go based off of pulse discharge rates. Some good ones are the LG HE2, HE4, the Sony VTC4, VTC5, the Samsung 25r, and there are several others out there. Check out some of the tests
@Mooch has posted. 2 batteries would be the minimum I think you should get, and you may even want to think about getting 4 batteries. If you chain vape all day, you will end up going through batteries fairly quick, and having a few extra spares never hurts. I always take my batteries out at about half a charge, and I go through at least 2 batteries in a night doing that(running a subtank mini at 30w). If I run all day, I can go and burn through 4 batteries draining them to half a charge in a day. Also, batteries aren't terribly expensive, so it doesn't really break the bank having a few spares.
For a charger, go for any of the ones mentioned by other posters. Personally, I use a Nitecore D2 and am happy with it. Part of me wishes I had gone with the D4 since I am using it to also charge my batteries I use in a telescope(and that can drain 8 AA batteries in a matter of 4 hours or so, and they take 4-5 hours per pair to charge, meaning it can take about 20 hours to charge what the telescope can use up in around 4 hours), but I have the D2 and make it work. If I were just using it for batteries for my mods, the D2 would have been sufficient the whole time.
For cotton, I bought a bag of Koh Gen Do pads. It came with 80 pads, and I peel those in half, making it like 160 squares. Out of those squares, rewicking pretty much anything takes less than 1/4 of the pad, so it basically means that 80 pack will last for about 600 wicks. Even if I rewick once a day, that is enough to last a year and a half. If you don't want to use that, you can use regular unbleached organic cotton balls.
Other tools that I recommend are at least a pair of tweezers(preferably with ceramic tips), a micro torch, and an ohm reader. After that, anything else is just sort of luxury stuff. Coil builders are probably nice, but I just use a drill bit when I make new coils. The micro torch is nice for being able to torch the coils and get them pre-formed before installing them.
If you aren't sure which coils you will prefer, you may want to hold off on getting more coils until you have given each a try and figured out which you prefer. Personally, I prefer the 0.5 ohm coils, and then I rebuild those with my own wire when they finally wear out. If you want, you can start doing that, too. I personally like the extra airflow the OCC heads let through compared to the RBA deck.
