Do these plug straight into the OEM harness, or do I need an HID kit?

They plug into your factory wiring. You'll need a separate D2S HID ballast and bulb set to power the projectors — the housings are just the optic part. Stock halogen bulbs won't work properly in projector lenses.

Are the angel eye halos powered by the same ballast as the projectors, or separate?

Angel eye power supply is not confirmed in the product specs. Test-fit and inspect the internal wiring diagram before final installation to determine if halos run off a separate circuit or share the D2S ballast line.

What prep or sealant do I need before installing these?

Use a quality automotive silicone sealant around the mounting points and lens perimeter to prevent condensation and water intrusion. Let it cure fully before powering on the ballasts.