Aerotaff, I was told by a good UK dealer that you can test the original steering pump for wear, and that wil give you some advance warning of imminent failure. My S1 had about 42,500 miles on the clock when I sold it and the wear was not excessive.
To test you need to get your hand down onto the drive cog for the pump. If you have more than about 1/2in of play (on the circumference of the cog) then failure is imminent. On mine at sale I had less than 1/3 inch.
Personally I'd stick with the original mechanical pump even if you have to get it repaired.
Cheers
