Your going to want a wet system for a 750+ cc 4cyl
Plenty of choices out there, NOS and NX seem to be the most popular and you can have either system for under $800. If you do what I did and design your own from various makers, the price will go up pretty quick.
Make sure you add a fuel pressure switch so that if you funn lw on the fuel side of things the nitrous system won't activate.
Back in '84 I didn't have such a switch on my bike and payed for it. Ever since all my bikes with the exception of my chopper and Goldwing have had a n2o system on them and I haven't had a lick of problems with them.
Also pay attention to the amount your going to be spraying. Rule of thumb is a stock motor can take 20 hp per cyl tops. If your going to hit a 80 shot of squeeze in your motor up your octaine or get yourself an ignition that can retard the spark to play it on the safe side. I run a dyna 3k and have hit several 80hp shots with no detonation issues.
Purge systems: on short line setups, its more bling then function. but they look cool.
Gauges: fuel, nitrous pressure are a couple good ones to have.
Progressive controllers: Your high end 7 sec bikes are using them. They are great for dialing in track condition. Some still like to use the happy button, but those that do tend to use a lower shot. In my old Turbo/n2o bike I went from a throttle activated 40hp shot and a 100 hit that came from the happy button. Once I went to a controller, I found I had more control and could dial the bike in to the track conditions. However, the way the controller works is it rapidly opens and closes the solonoids, this is very hard on some of them out there, so email and ask questions about the units your going to order. I just sold the pulsoids I purchased from noswizards.com in the UK, not the cheapest out there, but they worked fantastic with the controller. You don't want to have a solenoid break on you and either stick open (Nitrous) or not open at all (Fuel).