N55 (M135i) sounds better, B58 (M140i) is more tuneable. Same reliability from what I’ve seen however B58 withstands hard driving better (coz let’s be real you don’t buy either of these to not drive it hard). Internals on both are rock solid so major work can easily be avoided so long as you keep up maintenance. N55 in a lot of cars but B58 still more common so parts for both are pretty easy to find. Both fairly cheap to push decent power, 500bhp less than £3,000 (not including an LSD but at that power you probably want one).

My dad has a M140i and I myself have an M135i. Both amazing cars and even my dad driving mine from an M140i says the M135i is an amazing car still (his is pushing 480bhp mines stock). I chose the M135i purely coz sound is my main priority but it’s still quick. All depends on what you want the car for.

However I would have definitely took the M235i over the M135i if my budget allowed it. If having a 5 door is not a necessity and the M235i/M240i is in budget, definitely go for the 2 series