@J-gh 

Yes, you can use second Ethernet port on Main Deco to connect a TP-Link power line adapter to use with a computer upstairs. This should work just fine.

I have two recommendation for you, if you are choosing Deco mesh:

1. Consider Deco X50 or X55. These models are good compromise between performance and price. Check prices for both if both are available in your region: while similar by specs, their prices could differ (one model on sale and another is not, for example). Also, X50 and X55 have three Ethernet ports, you may find this handy.

2. If you are using TP-Link powerline adapters, you could connect Satellite Deco to powerline adapter upstairs, and computer to the second port of that same Satellite Deco. As a result, your computer will have powerline link, and also you can deliver WiFi to other devices upstairs with Satellite Deco having stronger link to Main Deco. That could be an option if you have good powerline link.