Oe fpr is usually has 1:1 raising rate, that means it maintains rail pressure when manifold pressure changes.
I use on my m103 turbo oe style regulator, its cheaper than china fpr and has reliability of oe part.

I think u can source Ms2 around same $ range, if u do swap from w210 then custom wiring will be much easyer than full swap from w210 for piggipack, u will have to get all modules to work. For overall custom ecu will be defenetly be cheaper.

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MB 300CE Turbo '88
MB 300E '86

Will definitely look into that too.