Official overall figures for 2005 cars
2.0 manual only 37.7 mpg
2.2 manual/auto 32.1/30.4 mpg
2.5 manual/auto 31.7/29.7 mpg
3.0 manual/auto 31.0/29.7 mpg
Later cars (2006-) are slightly more efficient, particularly with the replacement of the M54 6-cyl engine with the N52.
So on paper it looks like little difference between the 6-cyl variants, whereas the 4-cyl is much more economical. It is different in the real world where the 2-litre requires “thrashing” to get the best out of it; and my 3-litre delivers anything between 18-36mpg depending on how it’s driven.
If economy is your primary concern, though, an Z4 is probably not the car for you
The 2.2 auto on Autotrader under 50k at less than £6k looks nice, but expensive. Autos are not as sought after and this car is a very basic spec. You could do better, particularly at this time of year when convertibles are still at their cheapest.
Finally, don’t get hung up on mileage. These engines are capable of very high mileages (>150k not at all unusual) and easy to maintain. Buy on condition, spec, service history etc.