For the e39, I would recomend going to a dealer for the PPI. The techs there will still be familiar with the car and are used to doing CPO inspections on them. Ask them to do a CPO and you know every thing the car needs to meet BMW certified pre owned car. They will also be abe to perform a DIS test, will will encompass more than the aftermarket shop can.
I don't have any personal experience with SGS, but I not be inclined to trust them, since they are using "trained" inspectors, rather than mechanics who are familiar with that particular car and all its foibles.