What you mean with customize? How it looks?
I Made my last pc at alternate.de ... If it was a fair price, idk
mindfactory.de is rly good or atleast was.
You can register on pcgameshardware forum. I asked some ppl there to configure a good PC and I was not disappointed.
pcpartpicker.com is definitely the best. It finds the cheapest parts online and also makes sure everything you pick is compatible.
If you want a website to build a PC on and then let it find the cheapest parts across all websites, then go for pcpartpicker.com, they have a specialized german version as well
If not, then idk, don't know why you wouldn't wanna use pcpartpicker though
Build it yourself, this is so easy that even with low iq you can't do it wrong, also get more for each $$$$ spent!
Agando, reliable and cheap :)