The car's handbook will tell you what the max towing weight is fora 'braked trailer'. This is the legal weight that must not be exceeded. But I am sure that this will be higher than the weight calculated by the recommneded 85% rule. Using this rule the maximum loaded weight of the van, its MTPLM, should not exceed 85% of the kerbweight of the car. This is the guidance rule offered by the main caravan clubs to ensure that stabity problems don't arise. Although even here a badly loaded van can be within the 85% rule and still cause instabilty. I would weigh the car ata public weighbridge to asceratin its kerbweight and then look at vans within the 85%v rule and check the results against the BMW handbook for legal max twoing weight. Don't trust the seller of the van as some would say 'the BMW 330d would not even know it had a van on the back'. That's correct regarding its engine and gaerbox. Have aread on the two club websites for matching car and van.