With £10k to spend I'd say you've hit the jackpot narrowing your search to the Z3 & Z4 range - now you're only problem is deciding which is best for you.
I never owned a 2 seater car before 2018 and this year took ownership of my 16th Zed and 7th Z3 - a 1998 1.9 Z3 often referred to as the M44; my first was what is often referred to as the MCoupe (a 1996 E86) and I've had everything in between including 3 E89's and 5 E85’s.
Everything that follows is personal preference so please don't take it as a rigid guideline:
1) Are you looking to enjoy the car on long journeys with you other half - buy an E89
2) Are you looking for something to predominantly enjoy solo with a heavy focus on performance and handling - buy an E86
3) Are you looking for something similar to point 2 above but with a desire for a roadster experience - buy an E85
4) Are you looking for something similar to point 3 above but with a more nostalgic look and feel - buy a Z3
In short they are all great and if you buy well an enjoyable place to be. Also I wouldn't underestimate the benefit of forums like this and the ownership communities in general - they just add to the overall experience and serve as a great knowledge resource.
Most importantly have fun in your search, enjoy your purchase and remember the most important thing - with these cars rather than consider yourself as the owner it's more appropriate to simply act as it's current custodian
I don’t want both though
......maybe you've just not been here long enough 
