The pilot's failure to maintain an adequate airspeed margin while maneuvering in a steep banked turn to the landing runway, which resulted in an inadvertent stall/spin. Factors in the accident were: the pilot's inadequate preflight inspection of the aircraft in that he departed with the boarding ladder attached to the aircraft's exterior; the pilot's inadequate in-flight planning in that he flew a traffic pattern so close to the runway that it required excessive bank angles to align the aircraft with the landing runway; and the aircraft's probable encounter with the periphery of a weakened B-757 wake turbulence, which increased the wing's angle of attack beyond the stall point at a critical point during a steep banked turn.