The only thing I would say is ignore the tax bands. The little bit of money wont really factor into it when you take into account the car, fuel, parts. Just find the one that feels right to you. Check for obvious things like rust, grinding, clicking. Its still a car so a general look and listen should help and like you said yourself look for the Zed specific bits. One that got me was check the clutch make sure its still coming back up strong, if the clutch isnt returning properly the slave cylinder could be dying (what happened to mine) and thats a fairly pricey job.
Technically there are 3 model types and 2 trims. You have the DE and Revup engines which have the flat bonet and single intake. Some revups drink oil some dont but they rev higher. HR has the bulge bonnet and dual intakes and apparently has a huge amount of revisions and again revs a bit higher. Then you get the GT pack with the big bose sub and the heated leather seats, some have sat nav some dont.
But I would say one of the most important things would be the looks, find the colour you like. If you want it sunset orange with the orange interior find one and see if its what you thought in person thats the best advice I can give .