Cheap, easy and functional can be a difficult trio to hit, but that also depends on what your budget is, how easy do you need the system to work, and what function does the system need to perform.
A few questions first:
If they already have cameras and access control for each site, what is the operational purpose of these 1 or 2 cameras? Do you need continuous recording, event recording, time of day limited recording? Are these to watch entrances, lobbies, dumpsters, driveways?
You are correct that Avigilon Blue, Eagle Eye, Arcules, OpenEye (among others) require an on-site appliance. The advantage to this type of architecture is generally 2-fold: the gateway or NVR provides the secure tunnel through any firewalls and over the Internet to cloud services, generally removing the requirement for port forwarding or complicated network configuration. Also, the on-site appliance typically offers some video buffering on site in the event of an Internet outage. A couple of disadvantages are having an extra piece of equipment on site (cost, management, location, power, etc), an additional single point of failure.
The camera only, direct to cloud solutions like Stratocast, Axis AVHS-powered solutions, Axis Guardian (also among others) generally also can record to an SD card within the camera, in addition to the cloud. Configuration can be more complex because each camera has to be registered and connected to cloud services, compared to one NVR/Gateway, but this complexity will vary depending on the service.
There are also a couple of systems that provide camera hardware only with primary storage on camera using large SSD/Flash drives, and cloud as a backup from Verkada Cloud VMS/Cameras Tested and Cisco Meraki Cloud VMS/Cameras Tested. However, neither of these would generally be considered low cost, they both offer basic video analytics which performance was mixed depending on the scene lighting. Depending on your use case, they can be considered easy to use, but both are also limited to just a handful of proprietary camera options as well.
We have a listing of Directory of VSaaS / Cloud Video Surveillance Providers which lists out 20+ vendors that offer a mix of direct-to-cloud storage and onsite storage with cloud backup, with or without onsite NVR/gateway hardware.