Name of poster seeking advice: BugReporter
Which item(s) should they get, if any: Darkblood Stormking, then Arachnomancer, then ED
Why should they get it or not?: Assuming you mean farming the rep rather than buying with ACs: DBSK is the best class of the three and is the easiest to farm the rep for. Chunin and TSS have you covered for farming but you don't have any superb soloing classes, which DBSK can give you. Arachno would be next since it's the best offensive support class in the game and can do some good damage in its own right, so it's useful to have. ED is a good class but sort of outclassed nowadays and it's not that easy to farm the rep for (though admittedly Arachno is even harder), so it's not a high priority.
If you mean buying with ACs, probably just save your ACs. DBSK is easy to rep for, Arachno isn't quite worth spending ACs on, and ED isn't all that necessary.