ICM have put themselves in a bit of a quandary with this /6 version. Two of the markings sets are for Guderian's 251, but the kit has no Enigma machine, the tires have the wrong tread (needs Fulda rather than Continental), the radio set-up is incomplete beyond having no headset (or wiring diagram, a common omission), and the conduit for the wire from the radio to the frame aerial is missing. It also needs an additional "A" mount for the front MG (no shield), and the driver and assistant driver's viewports need to be lowered 1+ mm.
I think that MR Modellbau's MR-35600 "upgrade & stowage Sd.Kfz.251 Ausf.A (ICM)" is an expensive but essential addition for all the ICM kits.
The best source for the 251 variants is the old GroundPower Special, long out of print, but David Doyle's more recent book on the SdKfz 251 has some essential photos of Guderian's vehicle. There was also a 3-part article in the old IPMS Quarterly by Bruce Culver that can help if you have other sources to clear up a few errors and omissions. For example, I see the Enigma machine sitting on the front passenger seat with its pad removed rather than a specially built table based on the location of the operator's knees. It is VERY hard to find interior photos that show the details as clearly as we modeler's require.