'm looking for any recommendations on which cable modem to get. Is the one that Optimum provides gong to be good enough?
Do you know if this device is a "simple" modem? Or, is it a "gateway" device which is a combination "modem/router" in the same box?
A "gateway" or combination "modem/router" will require that the AirPort be configured in Bridge Mode....not what you want.
You should expect that it will not be possible to configure a "gateway" or "modem/router" to act as a simple bridge mode modem. Even if you might be able to do this, the service provider will likely not provide support you with this type of configuration.
Do you know the make and model number of the "modem"?
Generally, it is almost always best to use a modem that the service provider furnishes...to insure that you will get support in the event of a connection problem.
If the service provider furnishes a list of recommended modems....which they will support....that would be the next thing to look at. But, avoid a "gateway" or "modem/router" type of device.