I would defo go BMW... out of those options they are the best.
Fox will wonder if I'm crazy... but I prefer the 3/4 series to the 5... 5 just feels wrong to me - it has the cocoon of the 6 series, but in a saloon and the roof just feels too high and some other bits that just don't quite fit together. But that's personal preference and you might like it. Don't know if a 640d would be in budget? I rather like the 6 series.
Take a trip to a dealer and sit in them / test drive them / see what feels right with you.
I would have a poverty spec 320d over any of the other cars mentioned in this thread.
The CVT gearbox in the Lexus / Toyota is HORRIBLE... great idea in concept, terrible in practice.
I've never heard of a "magnificent 300C"... I only know of the "**** 300C".
Mercs from the previous generation are horrible places to be.
Golf/Sirocco R are very good for what thy are, but outclassed by BMW.
I haven't found any Audis that I like... but that's personal preference and I know that they are good cars, I just don't like them... the feel of the drive, no other reason.
I dislike diesel, but like Fox mentions - there is something good about the BMW 6 cylinder diesel engines.
I would still prefer petrol though... the x35i engines are brilliant and if you're gentle on the throttle then you can see 40mpg out of them on your motorway cruise (keep under 80mph and predict the road ahead) and still have more fun with them elsewhere. The x30d/x35d engines will see you 45-50mpg with relative ease.
Go out and have a look, see what feels right to you - everyone is different... but I love the build quality and handling of BMW, they're better than all the other cars in this thread. My wife's 224k E46 is still going strong and will easily hit 300k, I drove it today out of curiosity and it still doesn't feel rough around the edges, although we have done quite a bit of suspension/brake work in the last 1-1.5 years.