Will this fit if I've already modified my rear bumper or added aftermarket trim?

This bar mounts at factory points on the frame. If your bumper cover or other rear components have been heavily modified, mock-fit it first to confirm clearance with your existing setup.

Do I need to remove the OEM reinforcement bar to install this one?

Yes—you'll need to unbolt the original bar and replace it with this unit at the same mounting locations. It's a straight swap if you're keeping everything else stock.

What condition is this in—does it need primer or paint before install?

Ships as-is in new condition. If you want color-matched protection, sand and prime or paint before mounting. If you're tucking it under the bumper cover out of sight, you can bolt it up bare.