If you can brush your teeth without poking your eye out, you can probably install the ssk yourself.
Well, maybe not quite that easy, but it really is a piece of cake on the 128. The 135 is more difficult as you have to replace the selector arm as well (not to be confused with shift lever). Almost all the work can be done from the top. Only exception is the retaining clip for the shifter linkage. Even for this, I was able to reach up around the drive shaft from underneath without having to remove underbody plates or any other bits. As someone else mentioned in a DIY, the hardest part was getting the stock shifter knob off!