What can be said about the VAG TDI, its very reliable, if youre looking at the 8L shape A3 there a 3 engine outputs, 90bhp, 115bhp, and 130bhp, personally I would only look at the 6 speed 130s.
As for longevity its immense, 150k without any major problems is an easy target, mine has just passed 100k and it feels better every time I drive it.
The flywheel chatters a bit, dont mistake it for noise from the gearbox.
My TDI is a Quattro and that cause a few problems, some tuning parts for TDIs do not fit / work on the Quattros, however S3 / TT parts do fit.
I honestly cant think of any one thing that I would call a flaw with these motors, perhaps the smoke made when flooring it, and thats not too big a deal when you're leaving it behind is it?
If I were to buy another VAG TDI I could be tempted by either the VW GT TDI or the Seat Leon Cupra TDI, both 150bhp and fast as greased lightening without any modifications.
There are a few TDI / diesel specifc forums, not sure if it's the done thing to post their details in an open post so I'll send you a pm.