Thank you. I don't actually own an e9, I am instead considering it, and was wondering which modifications are "typical." Many of the cars offered for sale are not original, which is ok as I am not looking for a show car, but I was wondering what to expect as I look around.
Here is the priority list in logical order:
1) Get an E9 or you have nothing to upgrade. Worse, you will start collecting upgrade parts with no end.
2) 5 speed Getrag 265 from a good donor. At this time you will repaint the pedals, balance/cut the driveshaft and replace the guibo (pronounce goo-eebo). The clutch master cylinder will be a new part adaptation form a different model or a rebuild of a used E9 master. You will also install a new yellow pad in the transmission tunnel so you stop killing your neurons by breathing exhaust. It probably won't suffice as there are other exhaust paths, if you need to take an aptitude test for school or work do it before buying the car.
3) Now that the car shifts well you will drive faster and realize you need new shocks. Bilsteins worked for me. Some people refresh or use lowering springs at this time.
4) Now that you enjoy driving the car you spend more time inside and your neuron count is getting into lower numbers. You are still smart enough to realize there is fresh air outside if you could just open the window. This is where you will do the deQuincey window upgrade. You will need a Spanish to English Larousse to understand his labels and drawings.
5) You drive the car at night and when you turn on the headlights nothing seems to happen. Time to replace the sealed beams with Hellas or Cibies. There are two different versions for Euro vs. US wiring, but beacuse you are down to three neurons you will order the wrong one. Bummer.
That is it. Some folks will tell you to do a 3.5L upgrade with ABCDEFJ-Tronic or whatever.
Drive their cars and compare before you remove the carbs. If it is less than 25% better than yours just say no and spend the money on drinks.