When I bought my last diesel it was a 425 hp CAT the cat dealers recommended a cylinder cutout test......basically it gets the engine computer to run the engine on pairs of cylinders only by cutting off the fuel to the others, then a comparison of tach rpm to planned rpm gives a good test of how well those cylinders and injectors are performing. All in, IIRC it cost $250 and they claimed injection system and engine were in great shape. My experience showed that that was true.
the only engine related issue I had was the dash ac compressor seized hard and tore out the accessory belt. The belt was two weeks away and the ac compressor was four weeks away so we re-sheaved it and used a readily available belt to get it going the same day without the ac compressor.