@Argent Jones welcome to the forum!
You could buy as little as only one 24 and have enough to train for a year. I trained 1.5 years with a plastic 14 kg and a 24 (I would have been better off using only the 24 I think now).
Like
@Steve Freides suggested elsewhere, let what you are going to do dictate what you should buy. Have you decided what program you will follow?
Here the general recommendation is to do single bell programs for a while and only then doubles. Depending on what programs you follow you might benefit more from intermediate weights than from doubles, so no need to buy everything at the beginning. This would be my recommendation in order of importance.
- 24 kg (must).
- 16 kg.
- 32 kg.
- 20 kg or 28 kg or 2nd 24.
- Above 32. Or doubles.
Unless there is a very good reason for it, I wouldn't buy more than 24, 16 and 32.
About competition or cast iron: that's a matter of preference for most. I dont like cast iron for below 24 kg because they are too small. I dont like competition for 32 and heavier. Some fellas prefer the smaller handle of the competition for heavier bells because it helps their grip. Another good reason not to buy all at once, in the long run you might not like what you bought.
About brands: those brands you mentioned are all fine. I have bought generic Chinese bells and were just fine. For the first couple of bells you buy I would suggest a recognized brand. Some other brands might have some problem you dont know of, like a wrong distance from the bell to the handle, and it might hurt you not even knowing why (happened to me). So for the first few avoid this. Afterwards you'll know what to look for.
Good luck and let us know how your training goes.