I have all my church clients (and businesses) on Unifi hardware right now:
I don’t recommend the cloud keys as a lot of guys have had issues with them and they aren’t nearly as convenient as getting a $5/month Digital Ocean VM and spin up a controller there (or Vultr or Linode if that’s your thing). Anyhow, a USG will happily bang out close to full gigabit over the WAN as long as you don’t have a ton of VLANs because the US-24-250W is layer 2 only and all the inter-VLAN routing has to go through the USG (layer 3 Unifi switches are in development, and the new generation layer 2 switches will be released soon too).
Now, you absolutely need to plan for whether or not you intend to use the IDS/IPS when the 5.7 controller comes out of beta (likely soon). The USG won’t be able to cut it for IDS/IPS as it doesn’t have the processor for it, there is a new USG in beta but it is not even into the public beta release/early access yet, there is the USG XG in early access now and is soon to be released that will handle gigabit over the WAN with IDS/IPS and DPI (I suspect it will drop for around $1,500 but it’s really designed for 10G SFP+ connections to your switches), finally there is the venerable USG Pro that is rated up to 350 megabit over the WAN with IDS/IPS and DPI.
All the USGs have a bunch of features that include DHCP, DPI, VLAN, Guest Networks, RADIUS (auth and/or server), etc. Hypothetically it can handle giving out addresses to multiple /8 networks… I mean… you wouldn’t want to do that in real life, but you’ll never actually hit the limit. In the lab I’ve pounded one with a few hundred spinning up VMs and it wasn’t fussed. The old AP Pro’s tend to handle at least 30~50 clients each depending on what the clients are up to, I haven’t managed to choke an AC yet as I don’t have any running in environments with a lot of allowed clients, but it may not be able to quite handle the full hundred clients on a single one so bear that in mind when positioning them. Alternatively, the HDs can handle a lot of clients, or you can stick in multiple AC Mesh units in areas that are prone to high usage (you can directionally antennae those mesh units as well if need be).
My USG in my office will happily clock in at 950 megabits sustained download, and my UAP PRO AC will do 850 megabits up/down to AC devices. They are pretty impressive in that regard. Hopefully that helps with making the decision.