Do I need both left and right headlights, or can I buy just one side?

You can buy just the left side if that's all you need. If you want a matched pair, order the right-side assembly separately — they're sold individually.

Will this bolt straight in, or does it need modification to fit?

It bolts in at the factory mounting points. No drilling, no brackets, no modification — use your existing hardware and holes.

What about the bulb — do I need to buy that separately?

The assembly itself is included; bulb installation depends on your Corolla's year and bulb type — check your owner's manual or contact us with your exact model year for bulb specs.