Have both a 1.9 Z3 and a 2.5 E85 Z4.
Go for a 2.5 or 3.0l Z4, personally I prefer the Z3 but love the power in the Z4.
When looking for a Z4 check the roof works, has the roof motor been relocated to the boot, that's a good move to prevent it getting drowned in the original position. Check it has no warning lights lit on the dash, bad airbag modules are a common issue. Always best to take a scanner with you and scan the car for faults, if the seller refuses, walk away. Rusty brake pipes are also a common problem front and back, pay particular attention to where the rear lines emerge into the rear nearside wheel well where they get hit by spray from the wheel. Underside is well protected with undertrays but do take a look at the rear subframe area for excessive corrosion.