Welcome to the forum - a properly used Z, congratulations on finally finding us
Perhaps consider a stubby aerial - bit of a marmite subject on here though :lol: - also spacers front and rear are real easy to fit and will sit your rims out nicely in the arches. As for phone / music connectivity - with the CD changer and nav you should consider an Intravee 2 and Alpine unit. Compatible with iPhones / iPods, allows full integration with the audio system, real easy to fit, uses the cd changer connection that you can't use anyway, and allows access to lots of other things that you'd normally need to programme like side lights as DRLs, one touch 3 blink (or more) indicators, auto-lock of the doors and a bunch more. There was one for sale in the Z4 related items section at a good price. One of my favourite mods is the Gaptech one-touch / remote hood control that removes the need to hold the button down, and allows you to open / close the hood easily from the key fob. Also very easy to fit.
I'd better stop as I think I've managed to try and spend about £400 of your money in one paragraph