A quick build I did for my little brother. First time building in a small form factor build, too. He asked me to build a gaming PC for him for €650, this is what I came up with. I took the CPU cooler and case fans off my own shelf, so they were not inside the budget.

For the price, the Masterbox Q300L is an amazing piece. Great design, good cable management options and enough cooling options.
The Ryzen 6-core is plenty for his needs and paired with the 1660 really does the job very well, on his 1080p 144Hz screen (Acer XF240H). G-sync compatible, so the FreeSync works flawlessly with the Nvidia GPU.

He liked RGB, so I stuck with black components where I could.