I love the Da40 Tdi, it's my favourite SEP aircraft by far, yes they can be expensive during maintenance costs but they are a mere 19L per hour (approx) of Jet A1
I have maintained the DA40's with Thielert for over 6 years, I also fly them. If you are going to put some serious hours in every year the economy takes effect. In short, if you are not going to do say 300+ hours a year then the Tdi is definitely not the correct engine choice for you.
I'm not sure I understand the comment above about the G1000 in the TDi not getting updated, they get the latest software every annual as the Avgas equivalent does.
I have had a DA40 up at FL160 for 3+ hours and was pleasantly surprised how well it handled it, and if you get one with Autopilot they are great too.
If I can be of any help let me know.
Yodi