Keep the purge kit. Basically purges out the old nitrous that has been sitting in the lines to get a newer shot ready. You don't have to have it venting in the air for every one to see. I have mine venting on the back of the intercooler so it atleast is beneficial. Also the venting on FF2 wasn't nitrous, it was a CO2 kit hence the huge clouds.
Don't have it anywhere near your rev limiter. If you hit rev limiter and there is still nitrous injecting you will kill the motor.
It is important to watch the pressure but it isn't the end of the world. What he said is true, however you will never have the pressure at a dangerous level unless you have the bottle sitting out in the sun. As the bottle empties, the pressure will get lower. Hence why people use bottle warmers to bring the pressure back up. Simply means the hit from the nitrous won't be as big when the pressure starts getting low. I don't run a bottle warmer for the time being. Might get one in the future.
A gauge simply mounted on the bottle is more than adequate. If you want an electric one mounted inside the car those are available, however you can't convert your mechanical one into an electric one.