Bill you will love the change from an ST believe me,
7k will get you a nice example with the GT pack, which includes heated electric seats, bose sound system, cruise control. Also look for a car with the rays alloys, all uk cars had the brembo brake setup.
There are 3 models of Z and 7k will get you into the first 2.
03-05 DE engine 276bhp, most torque
06-07 Revup engine 296bhp, less torque but more revs then DE
07+ HR engine 309bhp more revs, more torque than Revup less than DE
Revups are known for drinking oil and from 05 onwards if I recall right its the higher tax band £400+
I wouldn't get to hung up on which engine to go for, yes technically the HR is the better lump but they all deliver a great driving experience
I would simply go on what is the best condition and looked after, take a drive in a few fella to get the idea of how they feel.
Good luck you will love it, and good luck keeping zeditis at bay