You already gave the answer, 15 inch alloys demand very low profile tires, making the ride very uncomfortable, and the steering, as you mentioned hard.
14 inch with 185/55R14H tires are a good compromise over the original, small 13 inch wheel/tire combo.
I have them on my '93 Fiorino Pick-Up 1,7 Diesel, without power steering, I can steer the car with two fingers, even at low speed.
I use a bit more pressure in the tires, and have 1,5 inch wheel spacers at the back.
Very important, the alignment should be at minimum toe-out!