A wifi extender will slow down your internet because it is half duplex. It can't send and receive at the same time. If you want to get around that, install a ethernet to the room with the weakest signal and install a access point(AP). If ethernet is not possible, and you have coaxial outlets in the room, look into MoCA. Either way, I suggest not getting a modem router combo but instead a stand alone modem and router. For the modem, the SB8200 is good but the Arris S33 is better if you want to future proof. As for the router, I like Asus but others here like Netgear. Do you have a price point in mind? Also, how big is your house?