Server resiliency

[Finlay15]

Hi. I'm new to hardware virtualization and I'm looking for some advice.
I'm setting up a VM to run a GUI-less Ubuntu guest.
The VM will have four virtual hard disks.

I would like some advice on the best configuration for flexibility and reliability.
The first virtual hard disk will contain the OS.

On the remaining three, I intend to setup LXC containers running Apache web servers.

I was thinking I'd make the first virtual hard disk a "fixed" disk since the size will not vary.
And perhaps make the other 3 virtual hard disks "dynamic".

Could someone tell me the advantages/disadvantages of making the disks dynamic linked?
The VMs will only ever run in Virtualbox, so I'm presuming .vdi format would give the best performance?

And I need advice for redundancy/resiliency. Can I setup something like RAID in addition to a backup regime?
Thanks in advance.