The base pricing on a Mazda 6 Diesel here is the same as an Accord Euro Sports Luxury (TSX with Tech) here. I highly doubt the 6 with the diesel would be affordable..
You're probably better off financing a MK6 Golf TDI. They should be getting rid of them for close to 0%, and they don't depreciate AT ALL. Most of the 2nd hand ones are really high in milleage without any price advantages. Also to note, the Golf is on a newer chasis than the A3 TDI, though the 'same' engine on the A3 has a unique counter balance to help reduce drivetrain vibrations (I still thought the golf was quieter)