When in doubt- by from MB.

I've use to purchase aftermarket, but my time is worth a bunch! I found that when you get factory parts at wholesale, they are quite competitive with the aftermarket stuff. I've gotten Lemforder stuff that was made in Turkey-complete crap.
Yes, watch prices- bosch rebuild starters/alts are probably identical so if there is huge price difference go aftermarket.

Just don't comprimse the reliability by saving a few $$. Did every suspension part in my W124 with factory parts and aftermarket Bilsteins. I'm very happy with the results.

Michael

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Michael McGuire
83 300d
01 vw A4 TDI
66 Chevy Corsa
68 GMC V6 w/oD
86 300E