DOes anyone know about fitting NOS or similar kit to a Vauxhall Astra?

Also, are they generally bad for the engine and what other components would need to be changed?

 

If they are set up right they shouldn't harm the engine, unless you put silly amounts in.
You might need an uprated fuel pump to cope with the extra fuel required and the ignition might need retarding abit. Depending on the state of the clutch it might slip.