In order to Future Proof your setup, you might want a DOCSIS 3.1 Modem, as others have said the more streams, the faster the service you can eventually subscribe to. 3.1 is backwards compatible to 3.0 in case Comcast doesn't support 3.1 in your area yet.
Buy a separate router. While it is nice and tidy to have an all in one, WiFi is approaching a new, faster standard. When 802.11ax arrives in the next 6-12 months, you could drop a new router in without having to replace the modem.
There are several factors that will be pushing ISP to offer faster speeds with the same, or lower prices. 5G is most here and the cell carriers would like to begin offering "Fixed Wireless" to homes at speeds in excess of 1Gbps, potentially up to 3-4Gbps over time. The cost of providing this service will be far less than stringing cables to each house, they simply spin up an antenna near clusters of homes and you buy a modem with an antenna to receive the signal. So, for the cell carriers, adding home internet service on top of cell, and offering video streaming on top of that (AT&T DirectTV, TMobile + Layer3, etc) and they can expand their services and revenue at the expense of cable companies.
What this will do is force cable companies to up their game to keep customers. DOCSIS 3.1 is potentially capable of up to 10Gbps down, 1Gbps up, so don't skimp on the modem if you think you might want to use these faster speeds at some point. ISP won't push those speeds initially, but the 1Gbps they offer in many locations now will likely come down in price and become available in more places. 3.1 modems don't all support 1Gbps, some max out at 300-500Mbps, thus the more streams supported will enable you to increase speeds without new hardware.
As these faster broadband speeds become a reality, faster WiFi will need to catch up, that is where 802.11ax comes in. It can potentially reach 10Gbps speeds, though initial offers will likely be 1-3Gbps. Current 802.11ac can reach 1.3 to 1.8 Gbps currently, but with faster broadband, you may someday want faster WiFi.
Before you dismiss all of this as over the top, consider that 15-20 years ago, we were happy using dial up modems at 56Kbps. Services will fill the voids when faster broadband is common. So, buying smart now can reduce your cost later.
As for the 5Ghz versus 2.4, I don't think there are any 5Ghz only routers. All that I have seen have both 2.4 and 5, and that solves 2 issues. Many older devices, and smart home devices use 2.4 because of the longer range, cheaper hardware, and adequate bandwidth for the limited amount of data they use. Plus, it will give you coverage over a wider area than 5Ghz.
While 2.4Ghz interference is more common, 802.11ax will also address this with methods to reduce conflicts. If you use the same network name and password for both bands, your devices will tend to connect to whichever will work better.
All that said, look at the mydeviceinfo link above to find modems that will fit the need. And look beyond the immediate need as it will all get pretty interesting in the next several years.
