Not sure about older wrxs if there are any components that need to be changed in order to be alcohol compatible but generally you will need a high flow pump and larger injectors as e85 needs roughly 30% more volume to make the same power.

If you want to be able to run petrol or e85 then it is preferred to also install an alcohol content sensor so that the tune can adjust boost, Ignition advance and flow.

I am sure there are others on this forum that know better and will add in some extra detail.