Hey Spokey!
Having test drove the 2.0i (150bhp) Sport and the 2.5i (177bhp) Sport back to back, there really isn't much noticeable difference in the performance between the two, the 2.0i does have a certain nimbleness that the 2.5i doesn't but of course you don't get that wonderful soundtrack. Having said that, around 5-6k revs it does sound nice. I get 38mpg average with mainly spirited driving around country roads. Running on 98ron petrol also makes a noticeable difference to performance.
I also drove a 2.5si (218bhp) Sport and that was quite a bit faster than the 2.0i.... quite noticeable and the one I would have gone for if it hadn't been £5k more...
I went for the 2.0i and I'm perfectly happy, and the newer ones with the Efficient Dynamics systems (2007 build week 34 onwards) average 40mpg.
If she was happy enough on the test drive 2.0i, then I reckon - like me - when you get used to the characteristics of the 2.0i engine you can learn to overtake using the right revs and gear and she will be even happier, but its no way as fast as what she's used to!
If you want something inbetween then the 2.5si might be the one.
I can only speak for Northern Ireland but dealers here are falling over themselves right now to sell cars, especially niche models such as the Z4. As one of the local salesmen told me "Its a buyers market, we are taking low bids on stock cars"..... Try asking your local dealer for an overnight test drive of each of 2 models to help you decide.
Good luck and let us know!