What are the choices: Buy new if you can find them, exchange for rebuilts, have a local shop rebuild them, take a chance on a used set, or rebuild them yourself.
For your 2 models of Mercedes it is possible to by rebuilt injectors off the internet for around $40 each (in the US) if you take the time to search for them. Added to that is the cost to ship them to you and the cost to ship back the old ones (cores) to get the money back for your core charge.
What did you get for that money: A set of injectors that are not as well balanced as the local shop would do and spray nozzles made in an unknown country.
Since the unknown country Bosch spray nozzles alone are selling for around $30 or more each the spray nozzles frim Sean C Watts are a good deal especially since you know what you are getting.
So compare the overall cost (don't forget the shipping) and QUALITY of buying rebuilts off the shelf and paying to have injectors done by the private shop with quality parts and I think you are ahead in the long run.
So lets say you supplied your own spray nozzles and you payed $30-40 each injector for labor (which I think is a little high). The local shop is going to do a better job on them as they know you are local and can come back for warrenty work.
You have to consider it an investment.

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84 300D, 82 Volvo 244Gl Diesel