We always replace pistons when we rebuild MB diesel engines; if they have more than 150k on them.
The pistons are available in .5mm oversize and the boreing to this size is about a quarter or less of what it costs to machine out the sleeves, press in new ones and rebore to fit pistons. As a result the cost to resleeve a motor and reuse pistons is probably more expensive (4x cost for machining, cost of sleeves, and cost of rings compared to: cost of single bore and cost of pistons with rings). BTW original rings are more than half the cost of pistons. Piston assy's are readily available from Mahle but all the separate ring sets are not original (Mahle or Kolbensmidt are original equiptment)unless bought through MB.
This info applies to 621/615/616/617 engines. I'm not sure whether the 601/602/603/606 motors have oversize pistons available.
With as much work as is necessary to do a rebuild getting cheap on the heart of the system don't cut it. It sure don't when it costs the same anyway.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician