Is there any risks of flashing the phone? Can it be permanently bricked or spoiled if the flashing goes wrong?

Flashing is not risky if uve got CID36 or CID 49 and NOT brown phones but you gotta follow all steps with pateince.Read all tutorials and understand before u get down to flash

If I flash my phone and change the system files, does my phone's warranty get void?

It does get void . but if u reflash back to k750 using tutorial 3 then ur warranty stands again ( since the sony guys wont know that uve flashed!!even if u fiddle with acoustcis, or camera, thers no way SE ppl can detect any changes. they just dont have the facilities to do it.