When I bought mine, I just arranged for the owner to take it down to a local BMW garage to have it inspected. I think that driving it there himself and the fact that it was a shop that dealt in BMWs reassured him. I told him that I was paying in any case and even if I didn't buy it, I would give him a copy of the report so he knew what might come up again.
The shop did it in a couple of hours, the fee was modest and they emailed me a report right away.
If the owner has no time to take it in, you can always ask if he could drop it off and leave it and consider using his shop that services it rather than an independent, although the latter would obviously be better.
No way would I buy a car like this without having an inspection. The days when I could do a satisfactory inspection myself are long gone - cars are just too darned complex today but OTOH they also offer easy hook up to the ECM for error codes etc.