The donor car is to provide the driveline for the conversion.  The 3.5 liter M30 was available in the 5-, 6-, and 7-Series, therefore all these models are conceivable donor cars.  The newer e32 and e34 cars are more desirable for the conversion.

Generally it is best to buy a fully operation car. This way the engine can be tested while it is still in the original vehicle and a determination can be made as to whether or not it is suitable for the project.  Also buying the car as a whole unit can eliminate the need to buy small parts that may otherwise be missing.  However, the principle reason for the use of a donor car is economic.  It is, in most cases, far cheaper to buy an entire car containing all the desired parts than to buy all the parts separately.

Older Models: e28 535i, e23 735i, e24 635CSi
In buying the versions that have a catalytic converter horse power losses of 33 total Hp must be accepted. This is primarily due to a lower compression ratio of 8:1, this "defect" is not easily remedied. Over-bored pistons would be necessary to increase compression. In any case, it is better to buy the 218 Hp version without the Cat.  These cars are available quite cheap, often they can be purchased for under $1000. The car need not be in good shape, as only the driveline is desired, so the price is often quite low. Best case scenario: a donor vehicle with a low-milage rebuilt engine is found.

The old 7-Series are the easiest to find and the cheapest to buy. The engines are, aside from very early version, the same as in the e28. Sometimes the older 3453ccm engines with the larger bore and smaller stroke can be found.

New Models: e34 535i, e32 735i
An e34 535i or e32 735i with 211Hp is the ideal donor car because the oil pan of these versions works very well for the 335i conversion. With this donor car the other costs that arise with other M30 versions are not a concern. The engine, despite putting out 211 Hp, is quite economical.  Prices for donor cars in this category start at around $2000.  The parts not used in the conversion can often be sold, to offset the cost of the project.  For the skilled salesman this is where the greatest savings can be found.

Questions, Comments?