To obtain the full megaman suit you must unlock a few achievments.

MEGA MAN BOOTS: Obtain the unbreakable achievment they let you keep the items in your inventory forever.
MEGA MAN BLASTER: obtain the zombie genocide achievment it shoots out real destructive blasts instead of the usual tennis balls.
MEGA MAN HELMET: no achievment needed it's just there to complete your outfit
MEGA MAN TIGHTS: obtain the punisher achievment im not sure what the affect is.

Note: after you unlock any of these items they can be found behind the airshaft where Otis stands in the security room.