Server Setup for Small Business

You don't necessarily need OS X server. Purchase a MacMini (or another iMac if you wish) and use that as your dedicated 'file server'. All macs are capable of being a file server and judging from your post that's all you really want for the amount of users you have. File Sharing is in the Sharing Preferences Pane. It's fairly simple to work out what you need to do.

A number of things you need to consider though:

1 Plan for potential power outages

2 Capacity

3 Back-ups

4 Redundancy

For 1 you can purchase a UPS and connect the server to that. That way data is protected from potential corruption caused by the 'shock' of a power interruption. For 2 don't just plan for the amount of data you've generated so far. Extrapolate 2, 3 or more years from now and don't forget the 10% rule. Ditto for 3. Make sure your back-up solution matches the capacity of your server. Don't just stick with one backup either, have two or even three, the more the better. I've seen multiple back-ups fail to restore when a worst-case scenario happens and you really need a reliable back-up to work. Don't stop there either, test your back-ups. No good backing up ad infinitum and when the worst happens (total failure) you can't restore anything because of data corruption or media failure. It's fairly rare but I have seen it happen. Point 4 is up to you as it involves more cost but you can't assume the hardware you decide to use is going to work 24/7 forever and a day. Have similar hardware handy just in case would be a good idea. The most important question to ask before starting out on this is: "what value do I put on my data?" If the answer is "it's worth everything as my business will struggle without it" then plan everything from that point onwards.

Hopefully this may help? Tony

Most of the features of macOS Server are being deprecated, including mail services.

Mail and other services are presently available on Server, though various of these will be removed with a future update.

Shared storage can use either a Mac or a network-connected storage device; what's called Network Attached Storage. macOS can serve its local storage to the network, and can for instance serve a locally-collected Thunderbolt storage array. Synology is one vendor of Network Attached Storage, and there are many others. You'll want gigabit networking, if you follow this path.

With the multiple displays connected to one system, you're envisioning a Linux system or some other classic timesharing system, and with multiple displays all being driven from one system. No Mac has enough expansion slots to drive all that; not with any sort of performance and any sort of resolution. Closest analog to classic timesharing these days is probably a Linux or Windows Server box or maybe a BSD server box with multiple thin-client displays. Those configurations very likely won't involve macOS, though.

One of the more common approaches for these sorts of situations in recent years is Microsoft Office365, the Google analog, or other similar software-as-a-service provider. That moves all your hosting and mail and most of the IT effort involved to the provider's servers and staff, which may or may not be desirable in your particular situation. This won't work if the data has to stay local for any of various potential reasons.

Beyond the soon-deprecated macOS with Server.app, local self-hosting options include packages such as ownCloud, and various others. Synology servers can also have add-on network services extensions, as well. I'd suspect some other NAS vendors have similar offerings, though haven't investigated those.

If you're considering running a local server, expect to have to allocate time and on-going effort to maintain and monitor and update and secure the server and the local network. NAS servers that are exposed to the 'net can and have been targets of attacks, for instance. Plan and budget for on-site and off-site backups, as well. Likely also RAID for critical data, though recall that RAID is not a backup strategy.

Have a chat with the business folks at the local Apple Store, and they should be able to provide you with some of the options and alternatives available when using macOS. Or scrounge up a consultant that deals in these cases.

you can't do what you want with physical displays, but unless there's some reason you want/need 8 separate physical displays, you'd be much happier with virtualization anyway. A product like VMware Fusion or Parallels Desktop is a very simple way to go and (provided you have enough RAM and disk space) can do exactly what you ask, with 8 separate virtual displays (i.e., 8 different windows, if you want to keep them open) to 8 separate virtual machines (VMs). Virtualization is a fantastic way to utilize the full capacity of your hardware, since many systems sit idle most of the time.

If you're feeling really adventurous, you can even use VMware ESXi to run the VMs w/o macOS. They'll run faster, but it means dedicating the machine to running only VMs.