Hello @AndyC350 ,

Welcome to our community.

Purchasing additional single units:
TP-Link powerline adapters are typically sold in pairs (KITs), but additional single units can be purchased to extend your network. Availability depends on the retailer and the specific model. We recommend checking with authorized online or local retailers, or visiting TP-Link's official website for a list of authorized sellers: https://www.tp-link.com/where-to-buy/

For guidance on selecting compatible additional units, you can also refer to: Adding Multiple TP-Link Powerline Adapters: How to Choose the Right Units

Mixing your existing AV600 with new AV8xxx devices:
We strongly recommend against mixing different series of powerline devices in the same network. Using different series (e.g., AV600 and AV8xxx) in one network is likely to result in performance degradation. Additionally, when pairing different models, the maximum theoretical Powerline Rate defaults to the lowest model's limit — in this case, a maximum of 600Mbps, which would significantly limit the performance of your new AV8xxx devices and may not be sufficient for your 1.1Gbps connection.

For the best performance with your new 1.1Gbps connection, it would be best to replace all your existing AV600 units with the same series as your new kit (e.g., TL-PA8xxx or TL-WPA8xxx series). You can use the filters on the TP-Link website to find suitable products: https://www.tp-link.com/home-networking/powerline/

Hello @AndyC350 ,

Models beginning with TL-WPA support Wi-Fi. Models beginning with TL-PA support Ethernet connections only. 

More details, please refer to Adding Multiple TP-Link Powerline Adapters: How to Choose the Right Units