That's quite a crowded WiFi spectrum..... Is it that busy on the 5Ghz frequency?
Do you get the full 240Mbps if you connect your PC to the router directly via Ethernet?
Getting 240Mbps over WiFi is going to require a pretty modern, 802.11ac router. Wireless N routers max out at about 900Mbps theoretical (300Mbps on 2.4Ghz and 600Mbps on 5Ghz normally). Since WiFi is half duplex, you can halve those speeds which means that theoretically you can get 300Mbps on the 5GHz frequency maximum. However, this is theoretical, so real world speeds will be much lower. What is the model number of your WiFi router?
Regarding powerline, devices need to pair to each other to connect, which involved holding a physical button on the two devices you want to connect. Once they're paired, then they're encrypted and no one else in the apartment could connect to them. As to how well they will work in an apartment, they should work fine. They work over the same wiring ring, so only outlets on that circuit would be able to even attempt to connect to the powerline. Without knowing how the wiring is done, I can't say for sure how well they will perform, but I would expect them to do OK. Make sure you get ones with gigabit ethernet, a lot of the cheap ones are 500Mbps standard, but only have a 100Mbps ethernet port so that extra theoretical speed is useless since you're physically capped at 100Mbps.
However, all this powerline stuff is redundant if you get really slow speeds with a direct ethernet connection to the router. If you get slow speeds when your network is:
modem-->router-->ethernet-->PC
then there is nothing you can do since the issue lies outside your home network.
