So I guess, first thing to ask - why Mazda 6? Lots of cars out there in that class...
To me, for a diesel, it should average around 20K miles per year. What type of journeys do you do? City driving only, then I wouldn't go for diesel.
What are you trading in? Would you try to sell privately - you would get more selling it yourself (but then the hassle of it) & get more discount as a cash buyer... I would recomend checking out the car web sites, cbg.ie, autotrader.ie, etc to get the price range for these type cars (if you are going for Mazda 6). Also, what guarantee are you getting with those cars, what is the history of those cars...