Reliability of the centurion engine in the DA40 is very good, most problems seem to be because of poor maintenance or operation.
The predicted TBR is around 2700hrs. (TBO not applicable).
Here is some fact-
Centurion 1.7 Event log
A few hours in the DA40 will make the transition to the DA42 very easy, the DA40 is a very easy aircraft to fly. It is very slippery and requires the pilot to think ahead, in my opinion, this is a good thing. The DA42 should be easier to fly then an early Seneca or similar where you have to be thinking about your approach during departure to stay ahead!
I agree that aditional training should be mandatory for aircraft fitted with the G1000. However some DA42's will be fitted with a more conventional panel (twin gns430/530's same as most IFR DA40's). There is an option of having G1000 fitted to a DA40, should this require mandatory training?