[Q] S3 Neo 19300I Warranty topic

So I am buying a s3 neo 19300I and I want cm 11 on it cuz touchwiz is kind off not my style. Is it safe to install a custom rom? What if I brick it ? What about this "KNOX"? They can probably easly trace that I installed custom firmware. But can I really brick it to where I have to take it to the store? Or I can fix that ? I live in Europe so its a normal version of the s3 neo not some duos ,lte or whatever.