Whilst trying to avoid getting involved in the existing scrap in this thread...
Before you start worrying about models, go have a look on autotrader at just what options you have. I did exactly that quite recently, i.e. looking for a diesel saloon with an auto box. The bad news is that there really isn't much choice. There's a small number of Saabs, VWs and Peugeots, the vast majority are BMWs or Mercs.
Bearing in mind the ability of someone like Merc to build an E class, that would be top of my list of cars to be looking at if you want to spend 12k.
Failing that, you could consider a Mondeo TD. I had a look at one very recently. I hadn't realised how OK, but nothing better these are until I checked one out. It was the perfect appliance. OK interior, OK to look at, cheap to run, probably very reliable. So if you focus is on just doing the job with maximum efficiency, it's the right car. If you want more, I'd look at an E.