I've had a fair amount of experience using nitrous on my prelude... check out the nitrous FAQs on the board here, and do your research. All the info you could really need is there... I also wrote a really good FAQ on PreludePower and T J... so check those out if you'd like.
A lot depends on what setup you're running, and what size shot you wanna use....
for a small shot, like 50 to 60, you're not gonna need to do anything at all. The motor, in it's stock form, is more than capable of handling all of that. It's probably capable of handling 75 too, with no real mods. At a 75 shot though, you're gonna want to swap to a step or two colder of a spark plug... and possibly a higher flowing fuel pump... although I dont remember having seen any of those for the S out on the market... but I'm sure they exist. The ignition system on the S2K rocks... so dont worry too much about that... and there's not much you can do anyway.
But obviously, with nitrous, you're gonna see/feel better results with free flowing intake, header, and exhaust.
Really, for a reasonable shot... I'd just buy the kit and put it on. Go with a bottle heater, purge kit, nitrous/fuel pressure gauges, and you're pretty much set. The motor is pretty strong, and capable of handling moderate nitrous shots all day long. Lemme put it this way... my prelude had 131K on it when I started using dry nitrous... I went from a 30 shot, to 50, to 70, back to 50... and took it to 174K with regular nitrous usage before I sold it. It never gave me a problem, except the stock clutch burning out.