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You asked about the frequency and cost of maintenance and repairs.
The 4-cylinder B230 engine variants as found in 740s and 740T/760Ts require more "short term" maintenance than the B280 V-6, such as periodic cleaning of the EGR system (actually a proprietary "flame trap" in place of the traditional EGR valve and associated plumbing), valve adjustments, and replacement of the timing belt every 50,000 miles or 80,000 kms. The B280 employs a timing chain in lieu of the rubber belt found on the B230. The chain should be good for at least 200,000 miles (321,000 kms) but is costly to replace. The B280 is also an interefence motor, meaning that if the chain fails, the valvetrain can sustain damage. The B230, on the other hand, is not an interference motor and its rubber timing belt can be replaced by the average shadetree mechanic at a very nominal cost. Both engines have front oil seals that are prone to leak on high mileage motors. The B280 has an aluminum block (B230 is cast iron), which necessitates replacing the coolant at least every two years to prevent corrosion. Valve adjustments on the B280 are easier than on the B230, but I believe that the recommended service interval is shorter for the B280. The fuel injection system is the same (Bosch LH 2.2) for both engines beginning with 1987 model year 760s. In general, these are easy engines to service. This is especially true of the B280 relative to contemporary American and Japanese V-6s.
Be aware that the 760 (both V-6 and turbo models) has Nivomat self-levelling shocks in the rear suspension as opposed to the standard gas shocks used in the 740. These are very expensive to replace. Some 760s can be retrofitted to accept gas shocks, and some cannot, depending on the model year. If the rear end of the car you are looking at sags, it's time to have the Nivomats replaced.
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