You need to do it right or not at all.
I got one at the shop right now. One of our techs rebuilt (ring job) the engine a number of years ago. He drove it a year and sold it. The car always had a ton of blow-by and used enough oil that eventually the new owner ran it low and ruined the lower end. He traded us the car for work on his 300e.
It sat a year and we got slow so we took itapart figuring that we would throw another crank in it and fly. I personally wasn't involved. After it got disassembled it sat and recently my assistant decided to put it together. My partner whined, but I refused to do it without new pistons. Here we were going to put $2400 worth of labor into this thing and for the lack of $300 worth of parts it would remain the same POS that it had been.
MB says that original piston to cyl wall clearance should be around .001 inch. The wear tolerance is .004in. Well I can tell you from many years experience when you have .004 wear it is not in a circle, it is oval and when you put new round rings in the hole they DO NOT seal.
You might get away with just rings on some gas motors but doing it to a diesel is just a plain waste of time (and gaskets).
Bosch Master, ASE Master, L1
33 years MB technician