Indexing Content Questions...

Recently I purchased several niche sites with existing content... much of that content (articles) have been widely distributed and are found all over the internet.

Of these sites, I uploaded (3) sites in total with their existing content intact, as it made it much easier to structure these niche sites the way I wanted them to layout & function.

Obviously, these sites have already been indexed, as has the content.

So, what is the best way to move forward...

- Should I just leave the content and add my new content?

- Remove/replace the content entirely?

Currently, I am (or was rather) in the process of replacing these widely spread articles with new original content, and wasn't sure the best practice for doing this (aside from writing them myself, lol)... so before proceeding;

- If I replace the article(s) completely (rewrite the entire post) for example and leave the post title alone, will the new content still get re-indexed?

- Should I decide to remove the existing content in it's entirety (*which is what I originally intended to do), titles included, what happens to the content that was already indexed, does it still appear in the SERP's, leading to a 404 error when someone stumbles upon the info?

(*which is why I questioned if I should just leave the titles alone?)

Being these are niche sites in highly competitive markets, my major concern is not necessarily scoring high with Google or relying solely on search rankings for traffic... it is more a matter of providing my (future) visitors with quality content, as opposed to regurgitated articles.

Naturally, any bonus traffic I can draw (in the future) from the SERP's is welcomed, but not my main focus here.

- Lastly, should I ever upload more sites with 'regurgitated articles' in tact, can I simple save them as drafts to avoid indexing?