Small Enterprise Hyper-V Storage reccomendations

Hyper-V Storage Recommendation for small enterprise required.

I have a customer that is a 25 seat accounting practice.  Currently they have a server that is 4 years old, dual Xeon CPUs, 72GB of RAM.  Storage is a RAID 6 array of 8 15K 300GB SAS hard disks.  There are 2 years remaining on the warranty.  The server is a Windows 2012 Hyper-V host.  There are 3VMs, a DC, a file server and a RemoteApp server.  The server provides more than adequate compute resources but is running out of storage space.  One of the file server volumes has already been moved to a Synology NAS connected via iSCSI.  Even with that, storage on the server is becoming severely constrained. Storage requirements are only going to grow as the client is moving towards "paperless" operation.

I don't know much about storing VMs outside of the box that they are hosted on, but it seems attractive to move the virtual disks to some sort of network based storage. Easily scalable storage, and when the time comes the ability to replace the Hyper-V host hardware and simply migrate the VMs without disturbing the VHDs.  For a smaller organization like this it seems (from my limited research) that a true SAN is not going to be financially viable.  Is it feasible to use NAS boxes to store VMs or am I going to be better off just recommending replacing the entire server?

DR is handled by a 4TB Datto Siris, so building a multi node cluster really isn't necessary either IMHO.  I know there are some small scale cluster solutions that I have considered, but with the Datto box in play, it doesn't seem like that's the route to take.

Your thoughts? Thanks for your time.