Hmmm. Why not just have the existing engine and transmission serviced and rebuilt as needed? Its much cheaper and your keeping the car whole rather than chopping it up. If your going to replace the engine and transmission why not just get a whole new car. Someone else can keep your car on the road and you can drive around in a lower mileage car.

I have always been uncomfortable with owners that replace whole engines and transmissions. Buyers often wonder what was so horribly wrong that the whole engine and transmission had to be replaced. Many buyers will assume that its a high mileage abused car. While at 400K your car has little collector value one never know's what the future holds.

With most MB engines the block's are known to last the lifespan of the car while the top end is what is generally overhauled. Realistically even if you put in a new engine and transmission a car with that kind of mileage is not going to be any more reliable. My 300SD has over 380K and still drives great but I know enough not to push my luck by using it as a full time everyday commuter car. Sort of like taking a 75 year old grandmother and expecting her to do an olympic marathon.