I got a 300sd with the stock alternator and an 800w amp powering 2 12s. Everything sounds good and I haven't had any problems with anything. I've been running this setup for about 2 years now. You should be fine I'm not sure how much power your headlights are drawing though but it can't be that much.

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W126 1983 300SD 286,000 miles and ticking
Baby blue exterior Grey MB tex
Recent work:
Replaced air cleaner mounting brackets and heat shields
Replaced alternator, fan and power steering belts
Replaced positive battery terminal
Replaced negative battery terminal and cord
New Duralast Battery

My car needs work.