I’m currently half way through what started as an engine bay tidy up for my SL230 in the U.K. .The cost will very much depend on what outcome you will the happy with. Shipwrights syndrome more commonly known as Shipwrights disease is the major risk. If you employ a specialist £20,000 was quoted to me.
In short ,Bonnet /hood removed ,engine out ,strip down the easy bit. Send all metal (except injectors and fuel pipes ) for passivation, powder coating ,painting or coating including bolts screws and washers .
Buy new injectors ,fuel lines. Buy new, all rubber, gaskets ,seals and filters . With the engine out arrange an engine bay respray. Send radiator for re core and renew hoses. The engines out so the ideal time to replace steering rods king pins and dampers .Arrange for the head to be re built . Send engine for measurements discover problems and decide to buy a new set of pistons . You may be able to come in at about £12,000 if you do some work yourself. Good luck, only other thing is allow lots of time .