The SLC-5/04 has a Data Highway Plus and a serial port built into it. That serial port out of the box is configured for DF1 communications. All you need then is a null modem cable like the 1747-CP3 cable you mentioned (besides a serial port). In RSLinx just stick with the RS232 DF1 Devices driver.
The serial port on the SLC can be reconfigured for other uses. One of those other usages is for the Data Highway 485 network that the older SLC-5/01, 5/02, and 5/03 had built-in. That could allow your 5/04 to talk with the older devices. If that serial port had been reconfigured for DH485 then the 1747-UIC would be used. Otherwise just stick with the standard 1747-CP3 cable.
As for the USB to serial converters, remember that a USB to serial adapter essentially creates a serial port on your laptop where your laptop didn't already have one built-in. You would still need a null modem cable like the 1747-CP3 mentioned earlier.
I generally tell people that if you recognize the brand name of the converter then it will likely work. If it is some off brand that you have never heard of, it may work, but those tend to be the ones that people have trouble with. Haven't used any from Radio Shack, but my guess is that it should work fine.
OG