1. There appears to be a few major players. Trendnet, Netgear, Zyxel, TP-Link and D-Link have decent products that sell for as low as CD$100 for a starter kit (2 adapters.) Here is a (somewhat dated) link to some top rated products https://www.smallnetbuilder.com/tool...powerline/view
Powerline adapters are like wifi routers. They have big impressive numbers that they don't deliver in actual use. AV2 MIMO looks like the best standard. Recent products come in 1200, 1300 and 2000 models. All will deliver similar performance with variations seemingly more dependent on make and model than generation. Expect about 400-500Mbps total throughput (up plus down) with one way throughput (up or down) at about half that. Factors such as noise, distance and circuit panel breakers can reduce that.
2. That should work. I would be tempted to set up a second router as an access point in the basement.
3. I wouldn't use a powerline adapter with built in wifi. There will be tradeoffs. Roaming within a wifi network can have issues. The most common scenario is that the router and device do not drop the existing weak connection and fail to connect to the stronger signal. Mesh can solve that but the mesh system and the device must both support the mesh roaming protocol. In any event, it will usually be no worse than not having a second access point. With a smartphone, turning the wifi off and on will usually make it connect to the strongest signal.
4. Kits with 2 adapters are usually already paired and are plug and play.
5. Extra adapters must be manually paired with the original kit.
6. I wouldn't purchase a mesh system right now. The best systems are tri-band and far too expensive. I also question whether wall pucks dependent on outlet location for placement is anything close to ideal. I've had good results with Asus AiMesh routers. They can be linked with a LAN cable or wifi. For example, one router could be wired on the second floor and another wired in the basement. That provides much better potential performance than a wireless repeater.
The obvious question here is, why not enable wifi on the Rogers router and configure the Airport Extreme as an access point? It's a no cost option and will provide adequate whole home coverage. Expecting one router to cover 3 floors is stretching the capabilities of wifi, especially on the 5GHz band. Apart from the roaming issue mentioned above, it should have no drawbacks. They can be configured with the same SSID or different SSIDs and channels to allow network optimization. Devices that roam can have both credentials entered.