I've looked some more into these realms and yes, they are just servers hosted by Mojang, so they'd be up 24/7. There is two versions of these realms, one for Mobile / Consoles and one for Java Edition. The limitation is that these different Editions can't be played together.
They seem to only supports up to 10 players in a server, which for us wouldn't be an issue? I don't know how many we have that would play? Some info from the FAQ;
HOW MUCH DOES A REALMS SUBSCRIPTION COST?
Realms has slightly different pricing plans for different platforms.
The cheapest is $3.99 per month for a server supporting up to 3 simultaneous players (including the owner) on the version that runs on mobile, console and Windows 10. It's $7.99 for a server supporting up to 11 simultaneous players.
The PC / Mac version starts at £5.59 for a server supporting up to 11 players. You'll get the best deal if you’ve set up a recurring subscription, or buy multiple months at once. Windows 10 doesn't allow recurring subscriptions."
CAN I EXPORT MY MINECRAFT MAP FROM REALMS?
You can export your maps from any versions of Realms at any time, even for a limited duration after your subscription has expired.
CAN I UPLOAD MY MINECRAFT MAP ONTO REALMS?
Yes, you can upload your own maps to Realms! We currently support worlds up to 5GB in size. That’s really big, in case you were wondering. In order to use a world template from the Minecraft Marketplace on mobile, Windows 10 or console, you should create a new world using the template, then upload the world to your Realm.
DOES REALMS SUPPORT MODDING?
The Java Edition version has support for a growing number of minigames and custom maps made by the community, while Realms on mobile, console and Windows 10 has support for Add-Ons and marketplace maps, textures and skins. However, full modding support is not yet available. Sorry!
CAN I USE ADMIN COMMANDS?
Yes. Owners of Realms on any platform will have access to most existing commands. On Minecraft for mobile, Windows 10 or console, Realm owners can also promote members to be operators, which allows them to use slash commands in game as well, but does not grant them any other admin rights.
Full info can be found here: https://minecraft.net/en-us/realms/faq/
So the most limiting factor could possibly be Mods? But would we really need Mods? You can get a free 30 days trial of Realms and even if only 3 of us was playing, the cost monthly split between 3 would only be £1.80. The more players, the cheaper it becomes.