Welcome to the Communities..
1)You can use VMware Converter to perform the P2V conversion and during conversion you will get the destination detail where you can select the ESXi host
2) If both physical and virtual will be online then there will be conflicts, If you do not want network connection to the virtual machine to perform your testing, then you just uncheck the connected and connect at powere ON option and use the vSphere Client console to access the VM.
3) If you are removing the physical server from the network, your VM will work as normal server and no changes required in DNS