[Big Dave]

The OM606 Benz might be tough to find. They were only used in late 90s cars and those cars are still serviceable and desirable.

The OW617 would be easier to find. Many more were imported and the early 80s Benz cars are starting to rust out. These are the undisputed kings of longevity with several documented million-mile cars using the OW617. but the little 5-banger is not the quickest thing - particularly at low RPM. Like any engine there are hot-rod mods and you could get maybe 170 HP and expect the engine to outlast the Crown Vic.

Panther engine compartments tend to be tight. The five cylinder might be the easiest thing to stuff into the car.

Benzes in Germany used a Getrag 4 or 5 speed manual but you'd have to source these transmissions from Germany. I don't think any Benzes (other than a few 240D) were imported to the US with sticks. Benz automatics -while durable - seem a little too loose to expect a lost of MPG.

Four cylinder Cummins engines are readily available and mate up to almost any North American transmission. But nearly all of them are super-noisy and they weigh about 800 lb.