Kayfun is a rebuildable tank atomizer "RTA" and can be somewhat finicky. Also the clones are not all equal and you could end up with a real junky one, bad threads, sloppy O-rings. You do need to learn how to maintain a kayfun, there are tricks to them.
A regular Rebuildable Atomizer "RBA" is just a chamber with your coils some wick and you drip onto it directly. A lot less to mess around with as far as building it goes but you do have to drip your liquid regularly. I would say I learned more and was able to experiment more with a regular old RBA.
Maybe someone could recommend a good single coil or dual coil RBA that is simple to build on with a good variety of air flow from tight to wide. As a mouth to lung vaper the wide open air of most RBAs these days is useless to me, I don't enjoy lung hitting at all. I really only had a junky IGO RBA before I started using Gennies, and then Kayfuns.
The IGO-L is pretty basic and cheap but it's a great easy RBA to learn on. Umm there seem to be lots of pre-wrapped coils on eBay. "Slowtech" (Fasttech.com) also has some though you'll have to wait a month to get them via the slow boat from china.
Well nowadays, we distinguish RBA's being defined as tanks such as kayfuns and foggers, and now RDA's are the drippers. Just a more recent lingo thing.
I'd suggest an EhPro kayfun or a Fogger V4 if you want to get your fingers messy. The Aqua isn't bad either. I either own or have owned all three and have thus far been most impressed by the Aqua, except for the low juice capacity. 3.5 mls or so if you fill it upside down isnt a whole lot. The Fogger will do about 5 mls, and I think Kayfuns are fairly close to that too.
The Aqua and Fogger are nearly identical. They're both intended for dual coils, which can be a little tricky at first, except you can adjust the juice flow on the Aqua. Not so with the Fogger. The Kayfun lacks juice flow control as well. All three have adjustable airflow which is a must. I'd spring for a Fogger V4. Their only 30 bucks. I got a black one because I think they'll look awesome with a black version of the new Provari when it comes out. A black kayfun wouldn't look to shabby either.
It would be well worth your time to learn to wrap your own coils. This makes it all the more fun, and lets you have that ownership of your own build and allows you to put your name on it and be proud of it. Experiment with a variety of gauges to see what you like best. I have 26, 28, 30 and 32 gauge, and 26 gauge is my favorite for my drippers paired with mechanicals, (best flavor and vapor) whereas I prefer 28+ gauge for regulated mods because it heats up a lot faster, but typically doesn't seem to match the flavor and vapor. However it would take ALOT of 26 gauge wire to be able to make something run on a Provari or something. I've done it with a genesis, but it took like 14 wraps and took FOREVER to heat up.
As far as wicks go, definitely look beyond store bought cotton. Rayon (a type of cotton) and Koh Gen Do (another type of cotton) seem to be getting the most attention right now. Personally I use Koh Gen Do and I really like it. No funky taste, wicks extremely fast and lasts a long time too. I got it on the sephora website.