There are 3 HP DL360 G8 servers, I think it's time to add storage, where to grow?

Question

Iron

For the purpose of introducing new services, adding jobs, improving manageability and reliability, we are considering options for expanding the hardware park.
Due to the considerable cost and strategic importance of the solution for years to come, we need advice and / or development of the solution by an experienced supplier / integrator.
Motive:

• Add to the domain users of the sales department as a separate structure, their mail on a separate MX-domain.
  
• MS Exchange redundancy solution
  
• centralization and increase of reliability of backup of files, mail, systems as a whole
  
• remote access to RDP via VPN (business trips)
  

What's available:
1. Hardware
• 3x HP DL360 Gen8 Server
• One CPU, one PSU, 16Gb RAM
• HP DSA B320i RAID (integrated, no writeback cache)
• 4x HDD SATA WD 2GB, RAID 1 + 1x hotspare + 1x noraid
• switches HP 1GbE x48 + 4x SFP
2. Services:
• Domain Controller Primary, WS 2012 R2
• Domain Controller Secondary
• Single domain, ~20 users
• Pair of small Linux Hyper-V machines
• MS Exchange 2013
• ~20 mailboxes
Propose a common strategy development (conservative, horizontal, vertical)
Suggest an option for the development of storage systems
• use of SSD as cache?, 10GbE?
Propose a virtualization application option that is optimal in terms of cost / manageability
Background: the investor suggested duplicating the existing hardware and software configuration. I think that so much money can be spent smarter. Integration guru, point me in the right direction?!
UPDATE: I was told in the comments that I should decide for myself rather than asking on the net.
I will paraphrase: there are three servers on SATA disks. DC-P, DC-S and MS Ex. What is better to do when the load increases: buy three more of the same or is it time to go to the clouds, pumping storage, buying a storage system? Tasks performed and added are described. All other information is given so that the question does not look so flat and uninformative. Thank you for your criticism and my reflection, but criticism is not an exchange of experience.

Answer 1

If you already have equipment and licenses, as well as a great desire to develop an infrastructure with new goodies, then you can consider the storage option:
- both servers will be upgradeable, namely:
+ buy and install + 48GB of memory for each server;
+ remove three 2TB screws from each server;
+ pokapat and put 2xSSD 480GB in RAID1 under the system, and swap out the remaining 2TB (repeat on the second server);
+ buy and install FC cards in each server;
+ buy storage with 12xSAS screws for 600GB (two controllers). 3.6GB I think is enough for you.
If this space is not enough, then you can immediately take it with 900GB SAS screws, or then buy a second storage system and include it in the perimeter;
+ connect both servers via FC to the storage system;
+ on one server you raise Hyper-V and already make virtual machines on it;
+ on the second server, we also set the Hyper-V role and from the first server we turn on the replica of virtual machines on the second server;
+ only the virtual machines themselves will be stored on the servers themselves, and hard drives will have to be located on the storage system.
Virtual machines:
PDC
SDC
Exchange Server
TerminalServer
Well, something like that!

Answer 2

Another option: In 3 days, Nutanix promises hyperconvergence on comoddity harware for everyone, three (4?) Nodes for free (Nutanix CE). Up to 4 HDD baskets, like mine.
Upgrade each server individually in 32-48 RAM, 10GbE LAN, HDD+SSD and stay on Hyper-V.