Nick, from reading your profile I doubt you have extensive experience working on cars since you don't own one. Older MB automobiles can be very expensive to maintain properly unless you do the work yourself. Before making a huge mistake be sure to have a PPI done. Remember there's nothing more expensive than a cheap MB automobile which has not had routine preventive maintenance done. Check if there are maintenance records for this 1990 300SE and have the tech review those when he/she does the PPI so he/she can make a comprehensive assessment.

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Fred Hoelzle