imo the major advantage of the step through is that you can run a rear basket and not have to swing a leg over it.
rack, basket and zip ties to attach are all inexpensive. Put things like pots of basil or a baguette in it to add style points.

my mums bike:

(baskets would be bigger but they have to fit through a series of anti-motorbike gates)

Attaching the upper stays with p-clamps is perfectly fine on your bike, but there are also lots of older racks which attach at the top behind the brake bolt.