Congratulations on the Job.
3 grand to spend, comfy bigish car. You know that 5 grand would get you a leggy S60 that's still reliable as long as you budget for a replacement ETM (now available rebuilt I believe).However, you want cheaper to run. Much as I hate diesel the real fact is that I'm saving 100quid every 1000 miles in my car, and doing the journeys just as quickly. I hate it for the noise but love the fact that 300-400 quid a month is going in my pocket instead of the governments.
So the obvious candidates are the better diesels - Volvo 850 Tdi, VW Passat,
2.2 Vectra (gonna be an old shape one), BMW 525d, BMW 325d etc. I'd go seeking out an old but looked after Volvo, BMW, Passat or Merc (in that order...) give it a good service and belt change and then enjoy. If you want something a bit newer post facelift S40 (2001 on) are getting to that sort of price level.Find an automatic one as well, there's no point changing gear yourself in a car that's slow and has a narrow powerband - let the car take the strain