I have owned the 1st Gen Z4, Boxster 987 and currently have a JCW Hatch.
Z4, not same model but ride 'overfirm' as you sit over the back axle and steering had no feel at all. A great cruiser but not a sports car but 2nd Gen I am sure is much better. I really like the look and you owe it to yourself to have a drive.
Boxster, mine was a 2.7 245 bhp - You have to buy the 'S'. The cars ability far outweighs the power and therefore will leave you fustrated. My Mini is quicker and that just cannot be right.
As an 'S' if you can live with a 2 seater and fancy some top down motoring this or the Cayman S are my choice. Steering is sublime, grip is imense, all controls have 'feel and weight' and just fall to hand that is far beyond more mainstreem cars. Get in you are driving a Porsche and as a car enthusiast for personal pleasure that to me has a feel good factor beyond the others. It is also a car you can enjoy driving slow as well as fast as you get feel at all speeds.
Mini, Great fun little car and I enjoy on a daily basis getting in the 'beast'.
It's not in the same league as a Boxster for drive involvement. There is a faire bit of torque steer in 2nd, 3rd but if you are looking for hot hatch fun it's up there for sure. Well balanced chassis, great down a B road and mine does 38mpg on the daily commute.
So from your list it has to be the Porsche and if you are at a time in life you can do it you will never forget the experience. The 1M would also be on my list and I saw a white one at the weekend and it looked fantastic. Gut feeling Porsche vs £40k 1 series the Porsche would win every time and just feel that bit more car for the money. You are driving essentially the feel they have put into a £70k 911 vs being based on a £25k hatch.