[The HACK]

Depends HEAVILY on the kit you purchase and install.

For example, certain StopTECH kits require that you ground down about 1/4" of the steering knuckle to fit their bracket. I did a 6 pot on an E46 330Ci and the process took much longer than necessary due to the need for the knuckle modification. 1/4" of cast iron to grind away with an angle grinder took at least 1/2 hour more than necessary. Don't know if that's still the case.

I also did a BBK on an E46 323Ci from UUC and that one was much easier.

I did a RacingBrake BBK on another make of car (Nissan 350Z) and that went pretty much straight forward except for the fact that we had to add one more washer to get the caliper to sit flush with the rotor. But again, that sort of thing is fairly common, the hard part was trying to find the right washers for the installation.

You will likely need to modify the dust shield so a cutting tool is recommended. A Dremel with cutting disc, or an angle grinder works much quicker than trying to modify with a cutter.

Also keep in mind, while you're swapping calipers, you should find something to plug up the brake line to prevent unnecessary air from entering into the HARD line. A golf tee works well.

LASTLY. Once the caliper is installed and the system flushed, take the caliper off and turn it upside down then bleed again, then re-install, bleed while tapping it with a large rubber mallet. Fixed calipers has more complicated hydraulic fluid passages that may potentially trap air. It's better to do the upside-down bleed and the mallet before you button everything back up, go drive and find that your pedal now feels like mush.

Other than that, it's typically pretty straight forward. If you have all the right tools, it's not all that complicated.