A:
This modem/wifi router combo is great. Your internet speed is dependent on the package you sign up for. This equipment is compatible with the higher speeds but doesn't speed things up on its own. If you've got a ton of devices, kids, or stream a lot - signing up for a plan that offers speeds of 50mbps (megabits per second) or more is suggested. 100mbps is awesome. But if it's just you and you have 1-5 devices in a small/medium house/apartment you could get away with any speed over 20mbps and be very pleased.
If the house is large and you have signal issues/slow speeds after your first week (despite being on a nice 50mbps internet speed plan) - look into a WiFi repeater or signal amplifier. (Like the netgear ac1200)
This Arris SBG 6580 modem/ router combo is great for all the below scenarios too:
I always suggest 50mbps plans for multitaskers, streamers and gamers. Families with more than 1 kid using 1 or more devices - 50mbps or more.