I made an answer for Appropriate preposition to go with "concerned", and noted that although I acknowledged that for could be used, I didn't knew in which context.
In the meantime, Dan Ray proposed an answer which give said information. Would it be proper for me to edit said information or part of it? The concensus on SO.meta seemed to be that it would be ok, as long as I give credit to the poster. But I can imagine that philosophy might change between the different StackExchange sites.
If you are referring to your answer that would be edited to include what reported in the other answer, then I don't see any reason for including what already written in the other answer. If you agree with what said in the other answer, you could add a note in your answer, saying that instead of repeating the other answer, although that could be a comment for the other answer.
It would make sense to add to your post what already said in the other one, if your answer is more restrictive of what already reported (e.g., "it's A when B; it is C when D" where A is what reported in the other answer) or for example, when the question is about the meaning of a word (or an abbreviation) that has four meanings, and the other answer reports just two of them. It also make sense if your answer is referring to what reported from a dictionary, when the other answer didn't have any reference, or when your answer is taking as reference a dictionary that was not referred from the other post. In the cases I described, you should be the one who answered for second.
I am not saying that what being the first to answer, and editing your answer basing on what reported by other posts is wrong, but I imagine what would happen if a third user would include what already said from the first answers, where the first answer has been edited to include the second answer. Users would read three different answer but that basically are the copy of each other.
If by "proposed answer" you mean the user has written as comment what should have been an answer (I cannot see deleted comment, and I am trying to guess what could be happened), then I would give the user time to write an answer out of that comment; if then the user doesn't write an answer, I would add what reported in the comment in my answer, giving credit to the user who commented.
This really needs to be determined case by case. In most situations, I would think it is impolite to copy an answer, no matter how much credit was given, because the rep system doesn't account for that. The exception is if there are a bunch of partial answers, i.e. none of them completely answer the question. In that case, the best answer would summarize the existing answers, giving credit where it is due, adding the missing pieces if needed, and — if possible — explaining why each partial answer is incomplete.
