This is a very nice car to drive, very comfortable and it has a lot of technical features. BUT, beemz are generally driven very hard by young yuppies. You're probably going to have to test-drive a few before you get a well looked after one. If you know a mechanic friend, take him with. otherwise take it to the AA for an inspection test.
Shop around a bit, bargains can still be found

You'll also find the same car in a smaller town at a cheaper price. Don't buy in the city.
For the same money though I would rather buy a Mercedes Benz CDI - the 270 is very powerful and will give you more torque and power. It's probably going to be a bit lighter on fuel as well.
It won't bankrupt you, but beem spares are more expensive.
Your normal spares, oil filter / diesel filter / air filter / brake pads are about the same as an Audi's - maybe 15% more.
When you replace wheel bearings, you don't just replace the bearing - you replace the whole hub. Last time I checked they were between R1600 - R1900 per wheel. The timing belt is also not just a simply belt change, you change a lot more. BUT, beem motors are strong and last long. My uncle has an older 530 (I think) and the engine hasn't been opened yet with over 550,000 on the clock - he clocked 550,000 2 months ago. The gearbox is also strong and could probably run to 600K before needing to be reconditioned - but expect to pay about R20K - R40K on that.
Depending on the condition of the engine when you get it, and how you drive, probably about 5L - 6L /100K
They're very reliable. Turbos are serviceable / replaceable items on all cars. Most newer cars' turbos gets replaced on about 100,000KM, my 2.8 Colt's turbo has been running 180,000K since the last service and we only replaced the impeller. If you drive the car hard, then allow it to cool down a bit before you switch it off. just let it idle for 2 minutes or so when you get home & change the oil regularly. Make sure the intercooler isn't blocked (after-market spoilers) or dirty and it should last you quite long. My mom's TDI Jetta had one turbo replacement on about 320,000KM so far.
Nice car, if you get a good deal on well looked after car.