I have only driven a short while a 335i, but owning a M myself I can chime in on a few things:
1) Average gas consumption is ~18.6 MPG on mixed highway/city. This is obviously very bad when compared to the 335i...and to top it off, you can already plan to spend even more money on gas than what you are budgeting! Why? Because you will enjoy so much driving it that you will record many more miles than your current average yearly drive!! Isn't that already giving you a hint on where I am going??
2) if you envision to keep this car ad vitam eternam, then you should get the Naturally Aspirated V8....which might become one of the last NA engines ever produced by BMW for the M (one never knows if the Engineers will succumb to the Turbo charms!). 335i will simply be a "common" car (no offense to this otherwise great car!!)
3) I expect the overall maintenance costs to be higher than on a 335i. Just take a look at the price of new spark plugs!
4) the financial deal is muuch better for the M cars right now... jump on these opportunities!
5) If money is not a factor, then you should not even ask yourself the question...
6) I fully understand which phase you are going through because I did the same for many months before pulling the trigger.... I have never regreted it so far. I am just frustrated during my work hours that I am not outside enjoying my sweet ride... All about the passion for the Motorsport, so CARPE DIEM!