Hi Chris
I too have a 1999 z3 without roll bars. Unlike most people, I like the 'without' look as it goes with the retro design of the car.
I bought and fitted a wind deflector from Just Roadsters last summer.
For me the jury is out on whether a wind deflector works for me, I love driving with the windows down and the deflector only seems to work with them up.
The wife definitely prefers it as it doesn't mess her hair up so much.
This deflector is designed so the roof can go over it, but I tend to remove it as it is easy to take off, just 2 thumb screws.
Good luck with what you decide to do. Go to some of our meets and cruises, then you can see other zeds and what they have had done.
https://www.justroadster.com/