I will cut the 123D story short as I think you have hit the nail on the head (and I agree with Akash in as far as the 3er would be a more appropriate choice), but suffice to say that it was a good car. In coupe form you wouldn't want to be in the rear for too long, but when is that an issue as the driver?! The 123D engine's performance, as I am sure you will know, is something else given it's a 2.0l, but I couldn't help feel that an autobox would be a better match as the first 3 gears are so short. That said, once you adjust to driving in a higher gear than you would typically in a petrol, you do get used to it. Economy is genuine 40/50 mpg (dependant on use) and has genuine ~150 mph performance - as tested on the continent.
Anyhoo, I had a hard time adjusting from the 6 cyl into the 4 pot and missed the noise especially, although over the last year or so I have Stockholmed and re-adjusted (Note: This is not a comparison, just my personal feeling). The power is effortless and smooth and you cannot tell it is blown, either in terms of delivery or noise. I had the auto which was great for shlepping along, but the 'paddle' functionality was a disappointment in as far as they are not
left for down and
right for up; also even in manual mode the box would not hold at the limiter. Performance speaks for itself and economy was far better than you would expect. To be fair it was about the same, if not better than the 3, averaging high 20's/low 30's. My biggest critism would have to be the M Sport ride: the combination of set-up and RFTs might be fine on smooth Autobahn, but anything less and things were less composed. I know there is a lot being said about swapping to normal tyres, but I never got round to that.
The last point leads me to something else you might want to consider...Alpina, either in
B3 or
D3 (the newer one having the 123D engine too). All the good bits, plus decent ride and something different. Or, what had to be the best of both worlds, the 335D.
Edit: The hyperlinks are no longer blue on the actual post?!.
Clickies on the B3/D3.