vSphere Upgrade from 5.1 to 6.0

Hi

We have a VMWare setup with 2 Nodes as below.

1. Physical Machine - Windows Server 2008 With vCenter 5.1

2. 2 Physical Hosts Running ESXi 5.1.0 HP Custom

We would like to upgrade the setup to 6.0 version with the conditions.

A. System is live & running 6 Virtual Machine, a minimum downtime is needed.

B. What should be the sequence for upgrade, first vCenter upgrade or ESX

C. Could we move all VMs to one ESX & upgrade the setup with 1 vCenter & 1 Node, then move all VMs to upgraded setup & then upgrade the rest & add to the cluster.

Kindly guide.

Thanks a lot!