Get the Optima Yellow-top or Blue-Top Deep-cycle battery (blue is marine version- supposedly more vibration-resistant).

Costco in the USA carriers the Blue-Top/Marine Optima. I have it installed in my 1988 300TE which has the same battery location & configuration. Just turn the Optima so the negative end (-) will meet with the negative/ground cable.

Also, get a Transpo adjustable voltage regulator and adjust so you have 14.2 VDC when you have your head-lights on.

A good automotive alternator rebuilder can get the Transpo (about $25).

:-) neil
1988 360TE AMG (OPTIMA blue battery)
1993 500E (stock MB battery)