If you are after improving spark then there is really not much to do. The factory coils are very high output. All the Bosch coils and wires for these are pretty darn good. I do use an iridium plug in my car mainly for the kv requirements to get the spark to jump with boost.
The coils for the m104 all update to one part. The originals ones were bremi or beru. And later went to the bosch. You can only get the bosch ones now which are the best.