Modern MacOS Server Restrictions So I see I can purchase macOS Server in the Mac App Store for about £20. This is all well and good, but very different to the way things used to work.

By purchasing the server app on the app store, do I get all of the features of the old server OS, for example Mac OS X Server 10.5, or does it only add a few features?

Is this a set of server management tools, or does it convert my client OS into a full server OS?

Can I use it on multiple machines simultaneously. If not, can I install it more than once (if I do not use it on multiple installations simultaneously)?

Is it a one-time-payment for all updates to the server OS?

is it limited to a certain amount of installs before I have to purchase it again?
It’s a one-time payment, and it’s standard App Store licensing — you can use it on all Macs you own for personal use which also means if you use the same AppleID to download to 15 to 30 Macs, you might get an email from Apple or just be in violation of the Mac App Store terms. 

Overall, the server app enables command line and guidance management of many server features already present in MacOS, since it’s Unix, and adds a bunch more custom code and server functionality from the app that packages scripts and binary tools as opposed to a new replacement OS.