Will this work if my Focus has parking assist sensors?

No. This assembly is designed for models without parking assist. If your vehicle has sensors in the hub, you'll need the sensor-equipped version to avoid warning lights.

Do I need to replace both front hubs or just one?

That depends on which wheel is causing the issue. If only one side is bad, one hub solves it. If both are worn, order two.

What tools do I need to install this?

Basic hand tools and a socket set. You'll need to support the vehicle safely and remove the wheel, caliper, and rotor. A torque wrench is recommended for proper fastener specs—check your service manual for torque values.