The Z2 is rubbish and will only display using proprietary WB emulation software but the Z3 (BLTZ3) is a different matter altogether.
I have had a Z3 in my A4000T for about 5 years now (even overclocked it) and it never falters.
I was lucky enough to be able to also buy the Retina auto-video-switcher which, when properly connnected automatically detects either Amiga video or scan doubled vide and send the correct one to a single monitor ( in my case a Mitsubishi 17").
With the Amiga's great ability to have overlayed screens of different resolutions it works ultra fast and works with P96 (I helped test the first P96 driver for the Retina) and CybergraphX V4.
If he is offering you a Z3 tyhen by all means take it.
Mike
Kyoto, Japan