Will this bolt straight onto my truck without any modifications?

Yes — this grille mounts at factory attachment points with no drilling or frame work needed. You're doing a straight swap.

Does it need to be painted before I install it?

The grille ships ready to bolt on. If you want a different color or finish, prep and paint it before installation. Otherwise it's good to go out of the box.

What's included in the box — do I need to buy fasteners separately?

You get the grille assembly. Since it uses your truck's existing mounting points, you'll reuse the original hardware that came with your Ranger.