Didnn't understand the question. You want to use the FreeNAS as a storage? Or you want to manage the ESX from the FreeNAS (???)
FreeNAS cannot "manage" vSphere - you can use FreeNAS for storage though.
Is this for a home lab setup?
As far as FreeNAS goes - your hardware setup will depend on what you can afford, how you want to set things up, and what kind of performance you require.
I have found this discussion useful:http://forums.freenas.org/showthread.php?9977-Poor-ISCSI-performance-on-FreeNas-8-3
I assume that you will want to use ZFS over UFS? Also assume that you will present whole disks as iscsi targets on FreeNAS rather than partitions? Will you mirror drives or create a zpool? How many TB of storage do you want for vSphere? Assuming gigabit network st least - and maybe jumbo frames? http://doc.freenas.org/index.php/Hardware_Recommendations
The phrasing of your question is confusing, and with so little detail it is hard to provide specific advise - hope that this helps.
I wanted to know if I could use a computer with FreeNAS to save and use virtual machines created by vSphere 5.x. If so, I would like to know what hardware characteristics should have this computer to be able to work with these VMs.
Ok. FreeNAS is not supported in any way as storage for ESX, but you can use it anyway fr testing purposes as an iSCSI storage or NFS storage (I would recommend the last one). You must plan the disk shares carefully (use RAID and good disks - i.e. SAS - for speed and safety).Depending on the amount of VMs you will store, you will need more RAM/CPU on this box, but there is no magic number for that - you need to monitor the box as the VM number grow. But, again, this is not supported for production.
So when I have to choose two drives (RAID 1 configuration), I must prefer those who have the IOPS reading and writing greater, and the value of lower latency. Is that correct?
For a workload of 5 fully active virtual machines that act as Servers, what is the minimum threshold of IOPS and latency?
The network cards of the NAS must have specific characteristics or not?