Using Original Content from Expired Domain Already Copied By Others

I bought an expired domain with great metrics and backlinks, and is very niche relevant. I want to add some of the content that was originally on the site, but googling snippets shows me that others have already copied content from this domain. Some is on forums and others on non-forum sites. The content on the site was very high quality and in-depth.

I'm guessing I probably shouldn't add this content, even though the domain I purchased was the original source of it. I'm using the domain as a money site because it's highly relevant to what I want the site to be about. It probably doesn't help that it isn't currently indexed either, so google won't know I'm the original source.

Thoughts?

You can use this content + add fresh content will be fine

No, you can't use this content. As you said, it's already indexed. No matter what that your site is the main resource in the past. But now you are not. So your content will treat as duplicate because you are copying it from the current index site.

Rewrite it and add more content