Components for powering led triggered by another light source

Hi guys,
I am looking at creating a light that is triggered by an existing light source. I don't want to touch the other light source wiring at all so I am looking at a photocell. The function I had in mind is when the device is powered up (on/off button) it measures the ambient light, it updates this ambient light levels in intervals and only triggers when there is a sudden change, when the existing light source turns on, turns on high or turns off. So the existing light source has three modes, off, medium and high brightness.

I am planning on using two or three 10w LEDs, I guess I need some sort of microcontroller, batteries for power, a photocell and from what I have been reading, a logic level MOSFET? I am looking to make this as small as possible but in the same time at a reasonable price and avoid excessive heat (I understand heat is unavoidable). I have been looking at this Mini Photocell. I haven't really looked at any LEDs yet since I don't fully understand the other components I need.

I am asking for some guidance in what kind of components I need to make this, I have been searching but got a bit overwhelmed about all the available components. Any ideas of a small microcontroller to use and what type of MOSFET? I guess I only need one analog input and one pwm output, right?

Thanks for reading!

A Google of "light sensor Arduino" should get you a few dozen hits. The most common light level sensor id likely a LDR (Light Dependent Resistor). There are also some ready to go off the shelf solutions like BH1750 modules. Detect ambient light levels and the rest id in your code. Use 3 digital outputs of an Arduino or just about any uC of your choosing. Yes, you can drive logic level MOSFETS or common transistors depending on the loads you plan to drive.