Hard call... personally I'd be looking at car 1 or continuing the search.
Car 3 sounds great but is still over-priced IMO despite the spec and hardtop (which you could always sell to get some £££ back). Is that private or an AUC car?
Car 1 sounds very cheap and the Z4 will handle that mileage with ease. It depends on the amount of miles you do yourself. If you do above-average mileage then you might be better with a lower-mileage car to start with, otherwise in a couple of years you could have a 100,000+ mile Z4 which psychologically will put future buyers off. But if you're only doing 5-6,000 miles per year, you could get a higher mileage Z4 on the basis that you'll add only below-average miles to it which should average it out.
Also, gannet raised the point that low mileage can indicate a car that's spent it's life mainly in town. That's not true in all cases although it is a valid reason, but it could be because:
a) the car's been a weekend toy: not always good either as it could sit all week, and only used for hard driving at weekends.
b) the car's just not been used a lot: this is like mine - not used for short journeys but also not just a weekend toy, I'm just sensible when I use it and would rather cycle than drive for journeys less than 5 miles.
For reference my Z4's a 54-plate 3.0i SE, purchased in Dec 2008 at £10.4k. I travelled 200miles+ each way to buy this privately from north-east England though, with the following spec:
- 18k miles
- 108s (since changed for 202s)
- silver
- not many options: no xenons, no heated seats (since added), no cruise (since added)
- immaculate condition: bodywork, interior, everything
- broken rear springs (price took that into account!)