Contact Phil at Parts Shop, give him your VIN# and he'll get you the right belt (you're correct there are 3 options).
You may want to change the belt tensioner damper while you're in there if yours has the external damper.
You will also have to loosen the tension locking bolt - a 10mm hex socket work great.
To set the tension line up the arrow on the tensioner with the markings while tensioning the belt with the 13mm tensioner.
Hope this helps.
1995 Mercedes E320