I would have to say the Gettysburg Address. If you want to focus on a appropriately-narrowed topic for an informational research paper, writing about this historical document I think would be the best route to go because you can find a lot of facts and write about the back ground history to why it was written, without going to deep into other events happening at the time.
If you talk about Abraham Lincoln, your going to get into so many different topics. You've got the Civil War, Gettysburg Address, the Emancipation Proclamation, other events, and finally his assassination.
How the Gettysburg Address inspired a nation to take action, I think would be more of a challenge to talk about because one will get into opinions and in a research paper, the teacher is going to want hard facts.
Why Abraham Lincoln and Marry Scott Todd got married, I think would again be challenging to talk about. I think it would be hard to find good hard core facts or evidence as to why this event occurred.
Hopefully, this helps.