In one rooom I have one PC with a router next to it. It's connected via LAN. Then I have another PC in another room. The distance between a router and the 3nd pc (in a straight line) is about 6 meters. That room is on the adjacent wall. Up until now I've been using a Strong's AP300 wifi repeater but I get high oscillations in download speeds and ping. Other than directly connecting my 2nd PC to the router via LAN, are there any other solutions you would recommend? Below is a drawing of how my situation looks like. I would try to avoid connecting it via cable as then i would have to drill through multiple walls.
Would a single pair be enough in my case?
Yes, if you only need to connect those 2 devices. You can always buy more units of the same brand and model and add them in later if you need and after you've tested with one pair if it works well.