The moral of the story is always to never buy what you can't yet truly afford.
Whether that means having to load up a residual that's so large that you're only renting a car, or moving into a house that is making every inch of your budget scream, just don't go there.
I wouldn't take a risk on a 3 Series, and especially not a 'boring' one like a 318i, and ESPECIALLY not an M-Sport variety (i.e. somebody pimped it because they can't afford a real sports car). If you're gonna hold a gun to your heard, at least let the prize be worthwhile the risk yes?
My advice would be to look at running costs. There are BMW run-flat tyres that cost R5 000 a pop and can only be replaced as a whole set. You get one of those cars and you're screwed.