@Goamute 

> - Which node do i choose to put downstairs?

Another Deco X50-POE could be a good option. Also, Deco X50 or Deco X55, if you could buy single unit. Not all Deco models are sold in single units.

> - Can i connect the node from downstairs to my router and also the X50-POE to my router? Or can only one node be connected to my router to make the mesh network?

Yes, you can. For that to work, you will need to configure Deco mesh to run in Access Point mode, which is recommended configuration when you already have a router. See Switching a Deco to Access Point (AP) Mode

>- Will the node from downstairs give a good and stable connections to my terras at 20 meter (i'm going to put a wifi-camera over there)?

A Wi-Fi mesh unit usually has a range of 9 to 12 metres (30 to 40 feet) through walls, but can reach 18 to 24 metres (60 to 80 feet) with a line of sight free of obstructions. 

Here is the thing: all WiFi routers have about the same range, except very budget models and pay through the nose commercial offerings. It is because maximum radio signal level is established by the government entity and anyone who is selling WiFi routers in that country must certify them with that government entity.

Which means, simply speaking, if your current router is WiFi router and WiFi camera does not reach it, place Main Deco near that router and camera won't reach it either. What you can do with WiFi mesh is place additional WiFi node in the house as close to the terrace as possible. If you have window facing terrace and camera, putting Deco unit on interior windowstill would be the best option you could have.

This Deco unit will become signal relay between camera and Main Deco. You may need to buy additional Deco for that purpose. If this is the case, buy two X50 or X55, in addition to one X50-POE you plan to, and you'll have complete set for what you want.