I would look at the quality of connections from the battery to the starter, including ground straps.

I found that after I replaced the dead starter on my '75 300D, years ago, with a used starter from an '80 something 300D, the cranking speed of the motor greatly increased to maybe twice that of the previous starter. It also started MUCH easier.

__________________
Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)