Your ISP should have a support page which lists the modems which are approved for your particular tier of service. Pick one of the approved modems. You are then free to choose any router.
Wireless throughput is a tricky thing. It is dependent on the router and the client hardware. If you buy the newest AC2500 router but connect it to a heavily used wireless G device, you will slow down your entire wireless network. If you live in a condo/apartment when there are many competing wireless signals your throughput will be poor. Move devices to the 5Ghz band if possible.