In one of HPI old issues, they mentioned that 300ZXs with VG30DETT is actually very good and reliable engine. There's nothing wrong with the V6 itself. The reason why most of them start playing up when modded is because poor heat management under the bonnet (not much room and air circulation), particularly in TT models. Therefore wiring looms will melt as they get old, and engine boosted higher than std would overheat coz it can't dissipate the heat, resulting in various engine failures over a long period of time.
If u want one, I suggest get a stock TT one, as young as possible (find something like 96 or above) & as low km as possible and when you modify it, don't be a superman. Try use sensible boost level & make sure you get either bonnet vents or aftermarket frontbar with extra hole at the front (I read somewhere the US version has this kind of bumper for better airflow).
Correct me if I'm wrong as I only know thinly abt 300ZXs.