It is illegal in some states, by varying definitions though.
Some you are not allowed to have the line connected, some not the controlling connected, some you can't even have a bottle in your car (not even to transport it).
It is allowed in a lot of motorsports though.
Using it on the roads is pretty silly, yet, "its only illegal if you get caught"
75HP is plenty enough for a street, stocker crossflow.
DO NOT use a dry system. Steer clear of them. I used/use a wet system. One spray bar for nitrous, one spray bar for petrol.
Leaning out an engine can
make it go bang, nitrous won't.
There are a lot of things that are considered 'best practice' when running n2o, but too many things too list in a post.
of the more important things are fuel pressures, colder plugs, timing retard, good fuel, etc.
Any more queries shoot.