Welcome to the site

As to the BMW 2.5TD engine.... ....errr, no comment!

Actually, I suppose I should comment.
Just because it has a BMW badge on it doesn't make it any good. In fact BMW stuff is generally poor quality, but thats a different story.
The 2.5TD engine in the Omega is the BMW M51 TDS unit. It is detuned in the Omega, making it a bit sluggish. A user here, Omegatoy, can provide chips which give better economy, and stronger performance. I have one in my (poorly) tractor, and it is thoroughly recommneded. In fact, the standard engine as supplied is utter rubbish - the chip makes it so much better to drive, and increases the mpg as well

If you get an auto, expct the gearbox to give up - Vauxhall fitted a weak box (AR25). You will need to replace this with an AR35, esp if you chip it (alledgedly, it was Vauxhall wanting to use the cheaper box is the reason for the detune).
Economy (hard for me to be too certain, as mine currently has a cracked head). Abysmal around town, excellent on a run. The average reading on mine at the moment is 40mpg. I'm not light footed, but it has done a couple of motorway runs lately.