Well...you're off to a good start.
1) You decided on a wheel style (design)/brand/size *CHECK*
2) You know what size tires you want on the car *CHECK*
3) You incorporated the drop of your suspension into the equation. *CHECK*
4) You want to factor in the caliper clearance for a future BBK *CHECK*
You pretty much covered the primary mitigating factors that can come back to haunt you later on. Good work.
I don't often see this much attention to detail with BMW owners. I have put tire/wheel packages together for a number of local guys. (BMW's and several other brands)
The general public is not always aware of the pitfalls that can arise when you change
one part on your car. That can cause you fitment issues if these factors are not accounted for in the pre-purchase phase.
Shopping for new wheels and tires can be a very happy experience...or your worst nightmare. (if you get it wrong)
In almost every case, the tire sizes will largely dictate which wheel width and offset you need to make it work without any issues.
I have always felt, that the wheel offset should
NOT be chosen in advance. (pre-determined) That
can and usually
does get you into trouble.
When you are upgrading to a larger tire/wheel combo, you want to build in some 'flexibility' in case things change. You cannot select a
firm offset number, and then 'hope' you can manipulate everything else around it. (to make it work)
I try to tell guys this is pure lunacy.
If you want the largest possible tires that can fit the car...then the offset by definition cannot the most aggressive fitment you can find. Those two things don't work very well...when you have a
fixed fender (or wheel liner) sitting there like a brick wall.
The front tire size you chose is totally unrealistic. A 20" 275 tire on the front axle is way too big for our cars, and you'd be rubbing on wheel liners for sure.
I recommend the following P40 wheel widths and offsets for your M3:
255/30/20* - 20 x 9 , +19 offset
*This is the largest 20" tire you can put on the front wheels without getting yourself into trouble. (clearance wise)
305/25/20 - 20 x 10.5 , +24 offset