I believe that most professional Nitrous company's will start you off with a 25BHP jet, see where you go from there, and if the engine can handle it possibly upgrade it to a 50BHP jet.
It really does depend on engine condition mate, as if your car has 150,000 miles on the clock and has been constantly missing services etc, then you'll be lucky if it can take a 10BHP jet.
But, as a fellow Alfisti, I assume the car has been well looked after, in which case I see no problem in a 50BHP Jet.
I have always considering on getting 'NOS' myself for a car, but I've not kept a car long enough to get around to doing it,
Who were you thinking of going to to get this done?