Where to place a Time Capsule

If I understand your current network configuration you basically have a roaming type network with the router in the basement and the devolo powerLAN device in the living room.

What is the model of the "Airport" that is also in the living room? Is its Wi-Fi enabled? Is it connected directly to the devolo device by Ethernet?

What is your goal in adding the new Time Capsule? Extending the Wi-Fi further? For Time Machine backups? For file storage? ... or something else?

The Airport is an Airport Express that is directly connected to the powerLAN. It's Wi-Fi enabled. However, the powerLAN adapter is also Wi-Fi enabled. Usually these do not interfere though I can't remember to have made a special setup.

It is never a good idea to have wireless access points near each other. The Express will attempt to use another Wi-Fi channel if it senses another access point nearby ... but it isn't perfect. Also there would not be any reason to have them so close to each other for the sake of Wi-Fi coverage. It may have just been simpler to directly connect the AppleTV to the devolo instead and left the Express out of the network configuration all together.

My goal is mainly to get a backup device for all my apple toys which are the mac mini, an old MacBook that I will "replace" with an iPadPro shortly, a MacBook Air and two iPhones. File storage would also be great added value.

May I suggest the following then:

• Leave the basement router as is. That is, keep it configured as your "main" Internet router and wireless access point.
• Disable the wireless on the devolo.
• Remove the AirPort Express entirely.
• Get the Time Capsule (TC). Connect it directly to the devolo by Ethernet.
• Set up the TC with the main router as a roaming network. In this case both the main router and the TC will broadcast separate Wi-Fi networks. However, since both will use the same network name (aka, SSID) and use the same wireless security type and password, wireless network clients will see them both as a "single" network.

Note: Time Machine only works with OS X or macOS devices. It does not work with iOS (iPhone, iPad, or iPod touch) ones. For backing up the latter devices, you would either rely on iTunes or iCloud to do so.

WLAN stability is sometimes an issue with the old MacBook but that will be solved anyway. Replacing the powerLAN which is about 4 years (200 Bit/s) old for a state of the art device (1200 Bit/s nominally ;-)) would also be feasible.

Yes, you will definitely want to upgrade your Powerline adapter(s) as the current one used will be a bottleneck. The TC sports 1000 Mbps Ethernet ports so getting the best connection between the main router and it should be a networking goal for you.