While some people report high oil consumption, this is not inevitable.
FWIW, my 2003 530 is now getting ~25,000 km per litre at 159k km. While my engine is not identical to the one you're considering, it is in the same family.
I bought the car with 111k km and who knows what brand or type of oil in it. The oil did appear to be clean & new. I have reason though to suspect it was the cheapest dino oil available. By 114k km I was down a little over a litre => 3k km / litre or worse
I was not a happy camper.Over the next several months I did three things
1) changed to Total Quartz 9000 Energy 5W40 which is a synthetic & BMW LL01 approved. An immediate improvement to more than 15k/litre. Others too, have reported that oil makes a difference.
2) renewed the entire CCV. I noticed oil consumption increased when it went bad enough to create drivability problems. After replacing the CCV, was better than after 1) so I suspect the CCV was creating some oil consumption.
3) replaced the OFH gasket, notorious for leaks. Probably minimal effect, mine was weeping rather than gushing.
I should mention too that I change oil at 10k km. It's possible that consumption could increase if I observed BMW's ~25k km recommendation.
The advice above to renew cooling system components is good. I'd follow it if I bought the car you're considering.
EDIT: someone above recommended a PPI. Strongly agree. A good mechanic may find lots of parts close to failure. In my opinion a PPI is the best money you will spend on the car.
