I am doing an OM617 in a 2nd gen pickup. The Benz is a good combination of cruising power and torque. Parts are available everywhere, including the US and it is a common engine. I would love to do a 1PZ(-T) or a newer common rail Toyota 4 cyl with all the great electronic control, but it gets expensive fast. I have done a 1HZ and turbo in the FJ62, so my expectations are realistic.. an HZ into a 2nd gen pickup is just too much. Lots of complete 300 sedans going for 1500-2500, and the adaptation to a Toyota drivetrain has been worked out. It is probably one of the cheapest and still enjoyable ways to live with a diesel as the Benz motors tend to rattle less like a tractor compared to others motors in that economy swap class of diesels.
Ultimately it depends on what you want. I couldn't find a 3RZ at the price point the Mercedes can be had for (with a bonus body to sell off). No matter what you do, you still have to do about 1000 in small bits and whatever it will run to adapt to a drivetrain.
Would love to do a twin turbo 1VD-FTE one day.
I really just like the simplicity of diesels over gas. The HZ has 4 wires operating it. Like that Toyota workhorse (longest run production diesel motor Toyota has ever made), the Benz is used all over the world in harsh conditions, like bush taxi in Africa. That gets points in my book, because I tend to use the trucks in harder environments and far from any population.