Thank you for the replies ! My first thought was for the 3700X too, and in the long run it make more sense.
I really like the U12s in black (and even more in Black/Orange LTT style, but my GPU is not RGB but only red so...), but I can use the stock cooler of the 3700X and wait for a better offer on the U12s.
Concerning the motherboard someone advise me to go for the MSI B450 Tomahawk MAX (its seems that it's one of the best B450 boards) and it's just 10 bucks more.
And I'm a little concerned with the RAM brand "Mushkin", never read or heard of it. And I thought that 3200Mhz was the sweet spot for Ryzen builds ? I don't know if 16Gb 3600Mhz CL18 NONAME make more sense than 16Gb 3200Mhz CL16 Corsair.