Doing the EP doesn’t really require much knowledge at all. What I would say is that you need to have the motivation to go through with it as my Sixth Form had a huge drop out rate for the EPQ. It helps massively if you choose to research something that actually interests you rather than just something that looks good.
As for marking, I didn’t think mine was the best and I got an A so it can’t be that bad. I presume you will do the type of EP that requires a presentation at the end which, although it doesn’t take too long, was the most stressful bit for me as I struggle to talk in front of a group of people. On the plus side, this is usually marked by your teacher so they should be fairly forgiving of you.
I think it’s quite strange chosing to do your EPQ at this later stage as I did mine at the beginning of Sixth Form. If you have the patience to write it in your free time and to sit through the writer’s block and do a few long writing sessions then hou should be okay. Just don’t leave it until the deadline day!
It can be very interesting and adds to your UCAS application or any other application you might make, as well as giving you transferable writing, research and presentation skills. And, like I said, lots of people don’t make it so you will stand out. Good luck if you do choose to take the EPQ!