If a 0.55 kW sorter section already runs PROFIsafe, does the drive need a safety relay?

No. PROFINET with PROFIsafe is onboard, so the 6SL3520-2XC00-5AF0 carries an F-address and joins the SIMATIC/TIA safety network directly — STO travels over the fieldbus. A cabinet-mounted safety relay and its wiring are eliminated, which matters on distributed lines where every enclosure adds cost and cooling load.

What overload and braking does a 0.55 kW belt motor need?

Belt duty is intermittent, so the drive is rated 200% of current for 3 s and 150% for 57 s. The integrated braking resistor handles decel without an external module, in a 380 x 156 x 129 mm FSA housing rated -30 to +55 C.

Why mount the drive at the roller instead of a cabinet?

At IP66 it bolts to the frame with no enclosure and no long motor cable back to a central drive — lower cable cost and no cabinet-cooling load for the line.