Do a quick search on "plug gap" and "M104" and I'm sure you'll find lots of info. If memory serves, it's .8mm. Also, the correct plugs and gap are located in your owner's manual.
It sounds like your car needs a thorough going-through. You should change all the fluids and filters, including the brake fluid and coolant. Ignore the 41,000 miles. Mileage is less important than time. That car is nine years old.
1998 C230 "Black Betty"