Thank you for your very usuefull awnser. I think I will stick with the stock cooler, and give the entire pc case a good airflow. I took apart the laptop like an hour ago, and ran some benchmarks with the bottom cover, and the keyboard removed. The temperatures dropped by 15Celsius. So I guess the stock cooler will also do, I just have to give it some more "fresh air".
Again thank you for your awnser, it was really helpfull.
No, I wouldn't recommend doing that. The GPU will bottleneck really badly due to the CPU. https://pc-builds.com/calculator/Core_i5-2400/GeForce_GTX_1060/0cN0Velu/16/
Also, depending on which model of the GPU u would like to buy, they use up to 80-90W when playing in 1080p.
Your power PSU would probably be enough, but you would be near the limits