Meanwhile I ordered and received a Volvo power steering pump. Cheap and low mileage unit, so should be good. I'll probably give it a short test in the weekend to see whether it spins up.
So far, I don't think it is going to fit where I wanted it to:
Alternatively I could fit it behind the front bumper. I have all the hardware to install the third radiator there, but am questioning the effectiveness anyway...
I'll need to have a further look into this.