I can't answer the question from direct knowledge, but I will hope you don't have to replace the whole unit. I priced "euro" headlamp conversion for the '85SD at approximately US$ 600. I guess I'll live with the US DOT sealed-beam units a bit longer, as they're about $15. Not as sexy though...
'96 E300D 58k mi (wife's daily ride)
'95 Audi 90 118k mi
'92 GMC Suburban 138k mi
'85 300SD 230k mi (my daily ride)