I have a 1993 E320, which has the same engine and transmission as your car.
I would guess that your transmission should last the next 100,000 miles.
I would suspect that if your head gasket hasn't already been replaced then it will probably go out within the next 20,000 - 30,000 miles. The original head gaskets were poorly designed and leak oil from the right rear corner (passenger side) of the head gasket. Our engine (3.2 liter M104) was used continuously in the S320 until 1999, and the head gasket was redesigned numerous times.
If you are unlucky, your throttle acuator might go out. It is not a very frequent repair item, but they have potentiometers that seem to wear out over time. They are expensive, about $1,200 for the part, and don't take longer than 1.5-2.0 hours to replace.
Other than those two relatively expensive repairs, their really isn't anything terribly expensive that I can think of. You might have a mass air meter or an upshift delay valve go out, but they won't break the bank.
Remember that even if all of the above need replacement, you are still ahead as far as money is concerned. Owners of cars tend to universally obsess over the high cost of repairs when they occur, but repairs are actually a very small cost of ownership - less than the cost of insurance, gasoline, and depreciation.
The biggest expense in car ownership is depreciation. Your car sold for $43,500 in 1996, and now it is worth probably about $15,000.
Your depreciation cost in the last 8 year is $28,500.
It would take a lot of repairs to equal that dollar amount, to say the least.
I'd suggest that you drive this car for as long as you can stand it.