As atinivelli already said, you need to have disks in the servers themselves. Apart from that you also need a supported raid controller if you're going to run vSAN OSA.
There are a number of other requirements that need to be met, like sufficient network speed. But like with anything, you need to have a good design first and then see if the hardware you have, could be used in that specific design. If not, then you're better of buying a new cluster if you want to go down the vSAN route.
