Stack Exchange is basically a big made-up game that drives people to compete or collaborate to provide the best possible answer to any question the site accepts. That also means the most complete answer, as every answer should independently fully answer the question. This encourages absorbing material left sitting around in comments. Absorbing material from other answers is also fair game: you're making a more complete and helpful answer, good job. On principle, everything's fine here.
Here's the rub: don't half-arse it. That's why you're feeling like you did something wrong. If you just add an inferior summary of another answer, your answer will look inferior and half-arsed, and you'll just be making the other answer look good by comparison. Don't update your answer just to keep up with the Joneses. If you're going to cover new material in your answer, make a stellar treatment of it that you can be proud of, and that we can be grateful you contributed. Beat the other answer by a league. If you don't, you're probably just dragging your answer down, and you won't be winning the made-up game.
You'll feel better for putting your whole heart and soul into making that amendment awesome. You'll feel okay for leaving your answer as-is.
Now, for crediting other users: that's only required when you're obviously using their work, because you're falling into the terms of our site's CC-BY-SA license. Otherwise, it's discretionary. Provide it if you feel like they deserve it. Make it short and snappy and out of the way. As a rule of thumb, if you're legally obligated to cite someone else under CC-BY-SA, you probably half-arsed it. If you did a really good job, you might want to thank someone for putting you onto it anyway, because you can stand proud that you worked hard and defer to the people who helped you get there without feeling like a cheapskate.
Also obligatory (not that you did this, good job): Don't signal your edits in text. If you're going to update your answer to cover more stuff, don't include an Update: section. That's being lazy. Revise your answer so that the update is just a natural part of it. It'll look better and flow better, and there'll probably be other parts of your answer that'll be more awesome for it.