Keep the modem and router separate, if at all possible. It'll be cheaper than an all-in-one, and if one craps out, you still have the other which reduces replacement cost further.
With Comcast, you'll need a DOCSIS 3 modem, stick with Motorola/Arris and their Surfboard modems. You shouldn't need to spend more than $65 on one through Amazon. Any low-mid-end router like the Asus RT-N12/B or TP-Link TL-WDR3500 should be fine, that should be had for under $40. Both support alternate firmwares such as DD-WRT which permits increasing WiFi broadcast strength, and should have the option to upgrade the antennas for higher gain coverage if need be.