Well, most decks separate from the tanks for building purposes, then you assemble them, and hopefully it's not a disaster. And an RTA, RDA or RDTA is going to be purchased separately from the mod, whatever you end up going with, yes. Depending on what wattage you want to run it at, and what type of coil resistance you end up using, you may do fine with an internal battery. If you wind up going for more wattage or lower resistance builds because you like them, you might find that you fare better with a mod with removable, rechargeable batteries, as you can carry spare batteries if you end up needing more power. It does require an investment in a battery charger, some good batteries (you can check out Mooch's blog for recommendations, if you decide you are interested, he is ECF's resident battery expert-- make sure you don't purchase fakes, or rewraps, and even vape shops are known to rewrap, so you want to go with one of his listed reputable battery suppliers.)
Also, if you are going to build, you may or may not want to invest in a Coilbuilders kit which will contain every tool you need for building coils. You can get one fairly inexpensively on Amazon, though not everyone finds that necessary if they have tools to hand and some experience with wire, electronics, etc. I found mine helpful, though I tend to be non-mechanically minded.
Steam Engine | free vaping calculators will help you figure out different wire options, coil options, and how to calculate your build so that it falls within your desired resistance level, but an ohms reader (comes with the coilmaster kit) can be useful to check the resistance, anneal the wire, and (for me) is also a nice, stable base to build on and wick on (you attach the build deck, I do that, without a battery) as it's easier to screw the coil onto the deck than if I'm holding it in my hands. You'll probably want to figure out what wire you might like to use, and buy some of that (though the coilbuilding kit comes with a bit of cotton and wire) as well as more wicking material (can be cotton, there are other options).
I think you are going to want to check out (and maybe post in) these forums:
Rebuildable Atomizer Systems
A lot of the pinned threads will give you more information, and a starting point, and you can "check out" some of the more popular atomizers to see which one might give you the best result.
If you do buy an internal battery now, that's completely fine, to get you started and if later you want a more powerful mod, and one that maybe does TC well, you do have some good options for a mod that will give you the option to use removable batteries, etc.
Good luck!
Anna
Anna