Bit of research suggests that the Z3 screen frame has a built in roll bar. Couldn't find whether it contributes to the structural integrity of the shell though.
Some interesting pics on this site promoting roll cages for adventurous drivers who don't seem capable of keeping the wheels pointing downwards -
http://www.bethania-garage.com/testimonials.htm
And none of their customers that sent in these pictures sustained life threatening injuries. Presumably the ones that did couldn't be bothered to write in.
As for keeping the build really simple, that's fine if you lack the mechanical skills to complete a more challenging build, but you will end up with a Z3 with a bodykit rather than something that looks convincingly like a fifties/sixties race car or classic. Each to their own
