The ZT is a better car than the ZS, comfier, quieter and more solid but they have a few niggles, the radiator fan relay can fail which can cause the engine to overheat when in traffic so check that operates, sticky door locks, ecu compartment flooding and a few more that I cant think of.
The ZS is a great car and doesnt really have any common problems, basically a honda civic underneath, and the handling is excellent the ride is very sporty so make sure your happy with that.
The engine is very good and can do very high mileage. You already know about VIS valves which are diy repairable if you can solder. econ isnt great (22-30mpg) and neither is 180bhp from 2.5l v6 but still a decent bit of power and low down grunt is impressive.
Cambelt change is the main reason why so many good cheap cars on the market at the moment, it is a £400-£600 job and not really diyable without special tools that are also expensive.
Another common problem is the thermostat housing can leak causing loss of coolant, you can usually see a puddle of water in the v of the engine (when cold) if it is leaking, worth replacing when doing cambelt.
That is all I can think of, off the top of my head.
I had a Rover75 (old mans ZT) with the KV6 engine and it was a great drive but my engine got cooked by the rad fan not kicking in.
These places are your friend for advice:
http://www.the75andztclub.co.uk/forum/http://forums.xpowerforums.com/http://forums.mg-rover.org/index.phpedit: just remembered the ZS can have weak clutch hydraulics but an uprated master cylinder is all that is required to fix.
