First thing to do-pull the cylinder head. Have a look at the pistons, if they are shot your waisting your time (a used engine will be cheaper). I have successfully rebuilt VW diesels that broke their timing belts, not a big deal. The Mercedes uses a chain, which is not as easy to set up but not impossible, but the internal parts can be expensive. It really depends what damage occurred inside the engine. You might get away with a new chain and related parts, head gasket and valve job if your lucky, but sometimes the pistons hit so hard the wreck the main/rod bearings too.