Congratulations on your choice to buy an Xm
The blue ebay 2.1TD is currently very cheap, and will no doubt give good economy. Probably around 35-40 depending on where/how you use it.
If economy is something your after, then a TCT petrol is probably not the best choice, these can be a little heavy on fuel, but perform well. I think most people get 28-30MPg on average from their Turbos? Then there is the 2.0 16v (in S2), these are very good engines, we have had three now. The 2.0 16v auto does around 30-35MPG on average, mostly closer to 35 and more if on a good run. Dont forget that this is a petrol and fuel is cheaper, always something to consider.
Though if you do a lot of miles a diesel would probably be better. A 2.1TD AUTOMATIC is possibly one of the most popular Xm versions, there seems to be always a few for sale at any one point, TCts are not hard to come by but the 2.0 16v seems the least common - going by what i see for sale.
Make sure the car you buy has had regular gearbox oil changes, its not unknown for them to fail and oil changes certainly prolong their life.
If you dont do a huge mileage, id possibly consider looking for a petrol, as diesels tend to be showing a fair few miles now and you might just get a petrol in better condition as a result. Though the diesels will do high mileages without little fuss and generally the Xm isnt a car which shows its miles.
If your wanting air con then your search will be harder, as this was only ever an option on anything but an exclusive, hence most dont have this.
Are you looking for an estate or a hatchback?
This looks excellent, 2.0Turbo CT, auto, exclusive:
http://www.carandclassic.co.uk/car/C137048/
http://www.carandclassic.co.uk/car/C129500/
Car and classic is always a good place to look for Xms.
Good luck with your search.