Basic vCenter and vMotion setup doubts - ESXi in different networks

Hi there,

I have a legacy vSphere setup I want to leverage and integrate into a vCenter, basically to be able to do vMotion and have all hosts visible from it, and I have several doubts someone with some basic experience could eventually resolve... so I will appreciate your help here.

I am more or less aware of the basic requirements to have a vCenter with hosts and vMotion working properly, however budget constraints does not allow me to do this at the moment.

Current setup

I have 4 hosts, in different IPv4 range/addresses, not physically connected some in the same network provider some in a different country and provider. There is 3 ESXi v5.x, only one is 5.5. And all of those have several VMs, 1 datastore each (hardware RAID).

Servers have 1 CPU with 4 to 6 cores each, 32GB  each except one that has 64 GB RAM. The servers have only 1 NIC each and it is not even gigabit ethernet as the provider charges a lot for that (40pound/month while host payment is 38pound/month).

What I would like to setup as for now is a vCenter that allows me to add those hosts and migrate virtual machines easily (even If I have to turn them off first), ideally, those would be in the same virtual switch so IP addresses would be maintained after the migration, however, I suppose with my setup that is simply not possible without VPN setups and so on.

Doubts

1. Can I actually have different hosts from different networks (not physically or logically connected other than via IP) working into my vCenter ? and do vMotion regardless the setup is not optimal?
2. Do I need another NIC for each of my servers as a mandatory prerequisite or would it work despite not being perfect ?
3. What happens if the vCenter is not available, what if the vCenter crashes and must be reinstalled, how do you manage or remove the ESXi hosts from the vCenter ? (I know you can setup so you can still login into the ESXi hosts but...)
4. Is there a RAM licensing that will apply ? where do you buy that if so ? I only see a "vCenter Essentials Kit Plus" on the website but no mention to vRAM licensing or anything. Was that an old requirement and new licensing is just more simple?

Thanks in advance