Few questions about Nitrous

Hello everyone,

I was researching N2O kits for my 91 Honda Accord when I came across this site and forums, so I thought I'd ask a few questions that were bugging me.. (yes I'm an N2O newbie)

In the very informative Nitrous FAQ sticky under the suggested items, it says..
Purge kit: Clears the line of “stale”(nitrous in vapor form) nitrous. Yes, you will notice a difference. Good to clear the line at the end of the night so you relieve the pressure on solenoids.
Remote bottle opener: You never know when someone is going to want to race you. It looks bad when you have to get out and go to your trunk to open your bottle.

Question, is it bad to open the bottle whenever you get into your car even if you don't end up using it? and if so, what is the max amount of time the bottle can be left open before anything bad happens? also how much do you have to purge to restore the power boost from "stale" nitrous?

I'm thinking about getting a Zex Nitrous dry kit just because it's the simplest kit to install.. but would a nitrous solution really be a safe addition for my stock(with K&N filter) 91 Accord LX (AT) with 160k miles? It's been well mantained with no engine work done and I believe passed a compression test in the recent past.
I'm also aiming for a 65 shot or 75shot if I can. Could I pull it off with my stock fuel system, injectors and ignition system? Or how much would corresponding upgrade parts cost?
No future mods planned and the nitrous will only be used sparingly for quick boosts when needed.

Thanks for replying!