It depends on your internet connection speed. If you're under 150 megabits down, you can use the SB6121 or a similarly-specced moded (4 up and down channels). 4 channels can handle up to around 170 megabits down.

If you're approaching 150 megabits down, you might want to consider a higher-end modem like the 6141. It has double the upstream channels, so you can get well over 300 megabits down from it.

I would use a separate WiFi router, simply because in my experience combo units don't work as well. It also means that if you want to upgrade your cable or WiFi, you can buy one cheaper device rather than a more expensive combo unit with features you may not use.

For WiFi routers, stick with industry standard specs and don't be fooled by proprietary add-ons. These usually require you use both a router and WiFi card from the same manufacturer, and that's not very common these days. N is the minimum I'd go for, but AC is becoming the standard. If you plan on buying a new laptop, smartphone, or tablet in the next couple years, you may see performance improvements in AC. It won't improve internet speed, but if you do internal things like sharing files between computers or streaming from a phone/tablet to a TV, it can help.

I personally have a Motorola SB6141 paired with an Airport Extreme. I used to use a Netgear WiFi router in the past, and it worked fine, too.