Conures are vocal. They make quite a few noises throughout the day, but happy conures don't screech that much really. The one in the pet store may have been bored or trying to get some attention from someone. Or someone did pay attention to it for a bit and then walked away, and it was trying to get him/her back. But, if bird noise is a problem, you don't want a conure.
My amazon is the least vocal, but he is capable of being the loudest. If he gets upset about something he does this very loud alarm sound that I'm quite certain can be heard by the neighbors even though I'm in the country. Thankfully he doesn't do it very much.
Well, I do think you need to think about the Rosella thing as far as how you would deal with it if it won't tame. But, I'm of the mind that when you bring home an animal it is yours unless it must be rehomed due to a death, illness or allergies. That's just my personal opinion though and no one has to agree with me. I won't be offended
I understand you are determined, but it does sound like it's a very difficult task. I'm a determined person, too, so I get it.