Having owned two n52-powered BMWs, I gotta say these cars are a ton of fun! They're pretty reliable for older models and offer everything you want in a sports car, even if you're not fixated on headline figures. The N52 is known for its reliability, classic i6 naturally aspirated torquey nature, and has a good following in all models.
For me, I'm all about the reliability - these cars are aging, so it's more important to me than outright speed, especially when regular road use doesn't allow any performance-focused driving where 0-60 is all that usable.
The 35is is like the cream of the crop, super desirable and very much coveted. But the N54 engine can be a bit complicated and might run into some problems which other models avoid. No matter which option you select though, the Z4 is bound to be a ton of fun!