Commutator, amature, brushes mainly inside.
If the engine sucked oil, you also have to clean it inside.

Note that some of the throttling valves have socalled tamper proof bolts. But these are also no problem to remove, just replace them with nice new ones.

Your dealer will have no spare part available for this, because this is 'tamper proof' secured.
I so far have never used a spare brush, just had to clean them.
The DK's are made by Bosch, so you may visit a Bosch shop for the brushes.
But I do not have a part number of that or a replacement.
That is no big problem, just measure it and find a replacement with same size. If you cannot find one, buy a slightly bigger size and file it to match.
Maybe Wuffer knows the part number of the Bosch brushes.

Then you can measure it, and make a function test if you want.
All nicely written down here
http://www.e31faq.com/attachments/att34.pdf