They are all strong, but the one likely to give you the most issues would be the diesel, that millage would put it right bang around the new turbo time.
I can't help but feel that the 3.0 l6 although a great engine could do with a bit more muscle for hauling the X5 around, a light vehicle it is not, it weighs a lot more than a 5 series, more than a 7 series even. So the V8 would suite it the best of the petrol engines, both petrol engines are liekly to give less trouble than the diesel, although I think the diesel unit suites the X5 very well indeed.
The only thing I would say is the V8 really will drink like a fish with the weight of an X5 trying to stop it moving. Diesels do make the most sense in these (it isn't often you hear me say this, very rarely in fact) ...but you'll probably find a better V8 car for your money (you didn't say what your budget was I know) unless you have about £15k to spend ...which I doubt as you wouldn't be buying one that old if you did I suspect.
As for what panthro said, that is wrong, the V8 will not cost you any significant (a tad more) amount more to service than the others and both petrol engines will 'probably' cost you less in repairs than the diesel is likely to. Fuel well yes, that's true, the V8 will drink a lot but I'd be willing to bet it wouldn't be massively more than the l6 in an X5 as it'll be working that much less to do the same job. The only one which will turn in anything other than 'bad' fuel economy is going to be the diesel, but then you don't buy these for economy do you
The only areas that the V8 is likely to cost you more in repairs is if the VANOS units need fixing (as it has 2, the l6 only 1) or if it has a slack lower timing-chain tensioner and has caused the timing-chain guide rail to get beaten up, that's a big nasty job.
Just listen very carefully for any rattles or tapping from the engine, at both warm and cold idle. Make sure you have a listen outside the car with the bonnet up when the engine has been properly warmed up, you don't want any tapping noises or noises like marbles in a can from the front of the engine (or anywhere for that matter).
![[TW]Fox](/data/man-of-honor/2009.png){kind=avatar}
Its trip computer told me it'd been averaging about 25mpg which wasn't too bad (no idea what kind of driving though, seemed like mostly around town work). 
