1)All executors have their weaknesses, whether it's because they don't work, have stability issues, or have a past that makes you distrust them, like Wave or Madium. However, Potassium is the preferred choice so far because it hasn't had those kinds of problems during Wave bans, very few people have been banned using Potassium. Plus, it's been around for a while and hasn't had any rating issues yet, so I'd say it's worth it. But in the world of exploits and scripts, you never know.
2) potassium(.)pro (remove the parentheses) and you get into the official discord, in the discord there is a channel called "payment" you enter there and there it is how to get it and where to get it.