If you have a vSphere Enterprise Plus license I would use Datastore Clusters and I would deploy a VM to a Datastore Cluster. The Datastore Cluster will select the optimal datastore based on capacity and I/O load.
Otherwise you can use the following PowerCLI script:
Get-datastore |
Where-Object {$_.Name -like "*NFS*" -and $_.FreeSpaceMB -gt ($_.CapacityMB * 0.25)} |
Get-Random -Count 1