I wouldn't do all of them at once.

Make sure your engine is nice and warm before you start as it will make it easier to get started again.

I always do one at a time and then start engine, stop and do the next one and so on.

Give that injection pump a good clean up before you do the delivery valve seals.

Good to see in your 'parts required' list that you have included the high pressure line plastic clips as they are pretty important-mine almost rubbed a hole in two of the lines due to rubbing.

I din't see any return hose for the injector return lines on your shopping list. Best to use the grey line with the orange stripe (3.2 x 7.0mm) size from Mercedes or OEM (contitech I think). I've tried all others even other German made stuff but they all leak in no time.

Were you working on or near something for the leak to suddenly appeared like that? How many miles/kms on the engine?

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1995 E300 Diesel