This is perhaps unknowable at this time, unless you are inside SpaceX, and they ain't talking.
However one can postulate on what is most likely. The basic SpaceX model is lightly built on Robert Zubrin's Case For Mars model. By lightly I mean the core concepts of ISRU, and using a direct Earth to Mars model. (Minor pause in LEO to refuel nothwithstanding).
That is, you send the return vehicle on the first launch window, and it contains the ISRU (In-Situ Resource Utilization) and power equipment. It sits on Mars for 2 years or so using its power equipment (be it solar or nuclear or fairy dust) and re-fills its tanks with LOX and CH4 such that by the time the second launch window comes around, the manned crew launches knowing it has an available return vehicle.
In that second window you also send the next windows return vehicle and continue to hop scotch in this fashion.
Therefore, it seems likely that power and ISRU is almost certain to be on the first mission. Without ISRU this does not work. ISRU needs power.
In addition, stockpiles of food are likely as well, since another part of the Case for Mars model is that if the initial return vehicle is disabled, no worries, the second one arrives empty, just wait out the time to fuel it up and leave. Thus food to survive through the wait for refueling is needed there as well.