RTA is easier.. just fill and go. (one comes with the kit) *and also, you wouldn't want a huge RTA since you are limited to power anyway. Don't worry about more power right now, you can get a more powerful mod later. The kit mod is great for traveling, its small and you can drop it in your shirt pocket or a cell phone case (not pants pocket, to much motion and risk of bending or contacting loose change) Remember, you said you had a budget, so I am thinking of terms of more bang for your bucks.
RDA you get better flavor and/or clouds. For example, I can put a 7 wrap clapton with a 4.0mm id core on an rda, dual coil. Those will not fit on most rtas that I know of. I drip usually once every 6 to 10 puffs just for vaping purposes. For clouds it is about once every 3 puffs. My build now, i vape around 170w but for clouds I up it to 200w. Same coil, different temp and air flow.
the extra RBA is like $2 for a clone a little more for authentic. It isn't required, but nice to have.
I think one tank (with the kit) plus 1 RDA (I would go with the MutX, since it does both flavor and clouds) and your basic building tools. As you learn more about building then you can get more. For example, I started ordering my 32ga stainless steel on a 30 foot roll. Now I am buying it by the 1000 foot roll. I didn't start out making claptons, now I do. You will need to get good at making single coils, and then try single core claptons. Clapton give better flavor, single wire gives better cloud.
I wouldn't worry much about batteries from fast tech, I recommend buying locally where they can prove they are authentic, first run and fully inspected and passed all quality tests.
The charger and spare battery are optional, but really nice to have on hand if your mod runs out of battery power. Just pop in a fresh battery and charge the depleted one... otherwise, you sit in a chair watching your mod charge and wishing you had a spare battery. LOL. You can always pick that up later too.
So the basics..
Mod kit with tank
28ga wire (and optional 26 ga)
(32 ga stainless steel wire - recommended but optional)
cotton
flush cut wire cutters
*ceramic tweezers - optional but recommended*
A battery, you may have to buy locally.
That's all you need to get started.
Once you figure out the cost of that, you can look at your budget and decide if you want to get something else now, or wait a bit. Like I want a new tank but needed nicotine for my DIY.. so I got the Nic now, and the tank I will get a bit later.
