Yes, all 112/113 owners of this vintage should really take note here. The service life of the sparkplugs is 100,000 miles OR 4 years. Lots of these cars were built in 97 and have been in there for the 4 years, and they do turn out rather hard after 4 years, did a set on a car just like yours last week, we charge book time of about 1.5 hours, depending on model (chassis).
In general what needs to be done is remove the engine trim panel (over the intake manifold), and remove the screws from the center of each coil on top of the valve cover. Pick the screws out of the coils with a magnet so you don't lose one, but DON'T ASK HOW I KNOW THIS!!!! Then the coil wires are removed. I recommend removing them just on the spark plug side, the wires will also pull off the coil side (by hand), if this appeals to you. Ultimately you only need to remove the spark plug side. You'll see there are 2 plug wires per coil, one over the other. The top coil wire goes to the FRONT of the engine, the bottom one towards the REAR. Also disconnect the small electrical connector from the coils. To get the metal end off of the spark plug, the recommended tool, per Mercedes, is a 17mm wrench which was originally used to adjust valves on an older Mercedes, the engine type escapes me now, as does the part number of the tool. The 17mm end slips into the indent on the end of the metal cover, and the wrench is then pried against the valve cover to pull off the end, works slick. On the ones that I have done, some of the plug wire ends weren't very accessable by this wrench anyways, and with some effort I was able to twist and pull the end off of the plug anyways. DON'T pull on the wire itself, obviously.
I recommend the Mercedes plug for this application, as I recall they retailed for about $11 each, you need 12 of them on a 112 engine (280 or 320), 16 on a 113 (430). Gap is 1mm and please do use a torque wrench to tighten, 28nm is the spec.