Cable wise don't bother with metal connector optical cables, they are not electrical so no metal is needed and the connection on the back of the device is plastic through and through, Using metal against plastic will cause undue wear and tear on the port, they're not that strong to begin with. A cheap amazon basic will do the job, 5 quid for a meter's length. Id certainly spend no more than 10 quid a meter.
Coaxial cables just need good shielding, here I spend just a touch more and usually a brand name for a little more peace of mind. I spent 20 quid 25 years ago, it was/is an ixos cable with fancy compression fitting on either end.
I currently use it from the CD player to my chord qutest.
Coax is better/preferable to Optical? (though I'm not sure why)
Where (if) a given optical cable is limited to 96kHz, coax will go higher - if that is important.
Optical would be the better choice for those that need the galvanic isolation (I certainly did), but Marantz to Marantz you almost certainly will not.
In your case, I would choose coaxial.
Coax connectors are the same as RCA.
A coax connector for your Marantz (and the majority digital audio in / outs) will be an RCA plug (more commonly and potentially confusingly referred to by older people as a 'phono' plug).
Any RCA interconnect cable will do even though it shouldn't.
You should use cable marked as 75ohm for your digital link - although, in reality, good luck hearing the difference between that and a non- 75 ohm coaxial cable.
Any cheap cable will be as good as one costing more but feel free to spend what you want? (Applies to both Coax and Optical)
'As good' provided the cheap cable meets a minimum quality standard for durability and, if you did choose to go with optical, check that it will be up to carrying your required bandwidth.
More expensive cables might have better connectors but unless you are often unplugging/plugging in this doesn't really matter.
Go with gold plated RCA connectors - it's beneficial and not a gimmick.
Streaming at 192kHz/24bit to a USB dac with SPDIF passthrough connected to the 1.5m RCA cable bundled with 100€ active speakers the receiving Pro-ject Audio DAC Box E (cheap Cirrus Logic dac chip - CS4344) works flawlessly.
Looks like ANYTHING will do, even for the highest bitrate.