im an owner of the 250 ohm version and i can tell you that my laptop, and even my phone can "drive" these headphones just fine, with plenty of headroom on the volume knob.
however, an amp definitely makes an improvement, and i think most people would recommend that you get an amp regardless of the impedance you decide upon.
iv never heard the 80 ohm version, but the general impression here, as i see it, is that these are the most bassy of the bunch. i imagine that IF theres a difference its a small one, but again, i wouldnt know.
so to answer you question, i think most people would tell you to get the 80 ohm version, and an amp. you may want to pay a visit to the zo thread in the portable amp forum though. i dont have a zo but i think its more of a sub woofer than an amp, but i may be wrong, and youll get a proper answer over there.