That's a bold move dropping down to half an engine !!!!. I haven't driven the 320d as I still couldn't get my head around a desial even though they are the rage in Europe. I surpose petrol prices don't worry me as I get a fuel card at work. Anyway, from what I'm hearing about desials, most (if not all offerings) are all well regarded these days in the motoring world.
As far as BMW reliability goes, it's no different to any other make out there. You have good and bad in everything. The difference is that BMW stand behind their product and don't try to wessal out of a warrenty claim. You will be dealt with in a proffesional and coutious manner and issues will be dealt with swiftly. Having said that, I'm onto my 4th 3 series now and can hardly recall the issues I had as they were insignificant. However, my 5 series I can write a book on but it's all good now.
Cost of services is on par with other presteige marques (probably a little cheaper than some) but it's no Holden or Toyota in prices. With my E90, it's telling me my first oil service is at 28,000Klm which I know you would almost be up to your second service in a Holden. I am going to get it changed at 15,000klms anyway.
Brake pads wear quickly so expect them every 30 - 35,000klm with rotors on each second change. If you like to keep your car clean, brake dust will be your biggest enemy.
Run flats have stiffer side walls but since you are going for the non sports pack, the ride quality will be very smooth. The sports pack while being very firm, is still very composed and makes the car retain it luxury feel. As for prices, around $480 for a 225/45/17 and $530 for a 255/40/17 (as used on the M-sport). You dont have to replace with run flat either. All you need to do is have a can of sealer and compressor. This is what I'm probably going to do when I need tyres as I think i've only had 1 flat tyre in 25 year of driving and even that was not a blow out.
You'll love it i'm sure ( as long as the lack of 200kw and a bank of cylinders doesnt worry you)