Ah. I have to admit it's an interesting roleplaying experiment. 5e heroes are way, way more powered up than BX. The spellcasters don't get tons of slots but they always have cantrips that will never run out, like firebolt. I would suggest do not use Feats, that will only complicate things.
But even if they had one action each, the armor could cast a spell, swing a sword and move. You would need to experiment some, for example, if one of the three parts didn't act, that would give the legs two actions to Dash (run). If the body doesn't swing a sword one round, the head can cast a spell more powerful than a 0-level cantrip.
That's kind of cool they can break up.