[WhatsADSM]

From one circuit track guy to another this is what I would change:

Use whiteline (or nothane in Aussie might be even cheaper) instead of M3 rear subframe bushings. They are essentially same in function but slightly cheaper for the part but most importantly VASTLY easier to install.

As far as arms. I would get M3 front tension and camber arms. In the back I would get M3 guide rods, and Megan toe links only. If you really feel like upgrading the upper camber arm go M3. Skip the rear trailing arm that will do nothing except actually add a bad failure mode to the rear if you hit something. Also skip the lower rear camber arm. If you swap to that lower M3 arm you will need to buy a coilover kit with M3 rear struts and 135i fronts. Keep the 135i rear lower arms and just buy 135i coilovers. Also replace the upper strut mount in the rear, many of use Monroe upper mount kit because it cheap (like $20 usd) and is much less compliant than stock . For coilovers I personally would get something other than BC. One of the Koni single adjustable kits, or KW, or ST coilovers kits will be only a little bit more money but much nicer set. Use the money I saved you above in arms and bushings to get nicer coils.

As for the questions:

1) Yes
2) Yes
3) Someone that knows the chassis well is always best. However the final settings really depend on the desired response, street ability, etc. If you just want to use a generic spec from the web (which is probably fine if you don't have a preference of your own) then likely a generic shop can do it. I would find one that has scales and can properly corner balance the car for you though.
4) As the above poster said. In terms of on track, tires, brakes, and suspension are all more important than outright power.

Edit: Some other thoughts. I honestly like to try to start with just a proper setup on the coilovers without messing with swaybars. Many times they are simply unneeded with a good set of dampers and correct spring rates. So again I would say take the money you plan to put in swaybars (which is a PITA on labor in the back) and instead spend it on a good set of coilovers.

Finally on the power front. Not sure if you have an N54 or N55 but in general a ECU flash tune will be smoother and more predictable on track than a JB4. I run flash tune only (MHD) nice and linear, smooth, and controllable. As you said it's all about control at the limit of adhesion on track.