Originally Posted by ward
because you'll actually be able to sell an M3 when you're done with it
I thinks that's right on. A modded a 335i will only make it's value more questionable, used BMW buyers usually run from even modestly modified cars, whatever out of pocket savings you gain up front you'd likely lose down the road. Also, I drive my cars for a long time, I'd be unsure about putting big miles on a stressed 335i.
If I were looking for a used 335i I'd not be interested in inheriting the previous owners mods, it's not a Honda or Subaru. On the other hand, M cars retain their value quite well, and retain with the potential to extend their warranty.