Mike, I have owned my 2001 M35 Stagea for 4 years and have currently clocked just over 203,000kms on mine.
The only issues I have had with it have been;
1. The turbo - it will die around the 100,000 or so km mark so expect to replace it.
2. Wheel bearing assembly passenger side
3. Radiator died a year ago after a number of 40+ degree days
4. Recently had a 0345 Cam angle sensor error which turned out to be a bend connector (easy fix, just a bitch to diagnose).
That's it.
I am still using the stock ECU which adapts itself somewhat to modifications and have been pushing around 1.2-1.3bar at high boost for the last 80,000kms.
Still runs perfectly.
The only common issues with these cars seem to be the turbo failing, usually around the 100,000-120,000km mark due to ceramic wheels and oil leak from the passenger side rocker cover (easy fix with a gasket from Nissan) and an oil leak into the rear spark plug hole (the seal is part of the cam cover) which is common on all VQ engines.
VQ25det was only used in the NM35 Stagea (2001-2004) and were only discontinued due to Japan's tightening of their emissions laws.
They are a pretty robust little engine and can certainly take a heap more power than they put out from factory, reliably.
Problem is that being an orphan engine, there is very little off-the-shelf gear if you are looking at over 250awkw.
There ARE some parts you can get from the US, such as a G35 radiator, VQ35 thermostat etc, but other than that is pretty much limited to sensors and the like.
Hope that helps.
If you need more info, look in the Stagea section or go to www.m35stagea.info