for the current diesel quality in taiwan. i think it is not a good idea to get diesel engine car. a lot of new diesel engine car need sulfer below 15ppm and i think the diesel in taiwan is still about 100ppm. i think that is the main reason why BMW taiwan haven't import 2.5L or 3.0L diesel engine car yet. the I6 diesel use latest technology and require better diesel.
in my opnion, for cars like 3 series or 5 series, the diesel won't save you a lot of money. some one mentioned that he can save over 100K when compare with 520D and 523. i think it is not a fair comparison. 523 is I6 engine and 520 is I4 engine. I6 is better than I4 no matter what you are comparing.