nitrous on a turbo car can/should act as a charge cooler... Not just more power from the nitrous but more power can be wrung out of the turbo too. Never done it myself but its well documented.

Only reason cars "blow up" with nitrous is bad installation/maintenance or use of some poor quality components and/or pushing the motor beyond the limits of what it can take - which would be true what ever method of tuning you employ. Try and run 50 PSI boost on a 125K mile bone-stock 1.3 Endura bottom end isn't going to last very well, but people have got over the fear of turbos which came from bad turbo installs in the 70s and early 80s which did blow motors up. Same with nitrous. Technology is moving on.

Some kits are more appropriate to small capacity engines common in UK / Europe than others. What works well on a 7.5 litre Chevy isn't going to bolt right up to a 1.6 CVH turbo...

How much gas you can run depends on may factors, but you can start small and work up, the jets are cheap enough. Your ignition system will probably decline to light it all off before you get to the point it does damage (assuming a properly built turbo motor).