Hi André.
Well no actually, I don't boot from USB but indeed from one of the hard drives. There is an onboard USB port too but I can assure you that it's not in use.
So I am very confused about what is happening.
I just booted this machine from a live CD because in order to make sure the VMs are still there, and I could see again the bootable partition when performing fdisk -l from it. As for the rest the fdisk -l output looks like usual but of course I cannot mount what I'm guessing are the vmfs volumes without vmfs-tools.
What worries me the most is that I can't see anything in the 'vmfs' folder when accessin the ESXi console..., it's empty, no 'volumes', nothing. In a way I prefer this to showing me empty disks, but still, it's very unclear what's going on with the disks at this stage. I could hardly believe that the data could have been erased tho, just through rebooting on a misdesigned oem.tgz file...
Following this link about collecting disk and LUN information I invoked esxcfg-mpath -b and it returned nothing!
I am in the process of getting my hand on another live CD shipped with vmfs-tools and follow the instructions from this page.
Yes I agree with you about purchasing an affordable NIC, it was a mistake to get down that path of pushing for unsupported drivers 
However if anyone has any idea as to how to restore my ESXi to a working state... I'd be, as you can imagine, very grateful.