Frankly wouldn't worry about mileage, I bought my daughters 2006 1.6 mini one with 122k on the clock 18 months ago, it has now done 143k. I am pretty confident she will get 200k out of it. Now take into account this is a small engine that will be worked hard. I would expect a Z4 particularly a 6 cylinder to last much longer. It can be more problematic buying one with low mileage particularly if the journeys have been short as the oil can end up emulsifying and if not changed regularly can build up in the oil ways.
Two colleagues have just bought 3L Z4s with 102k and 105k late summer and they are going fine.
Regardless of mileage you need to make sure they have been well maintained, particularly oil changes and depending upon driving/routes be aware at around 100k you might be getting close needing a new clutch if it has been driven on routes requiring a lot of gear changes, but I would expect the engine and gearbox if looked after to go way beyond 100k.
