N2O can give you short term gains of anything from 50bhp to a couple of hundred depending on the size of the jets used. I used it on a Sierra Cosworth that I ran at Santa Pod years back. It was already pushing 570bhp and I had an additional 80bhp from the Nitrous. I never used it on the road though, the gain was just a short term 'squirt' of power.
If you're hill climbing I'd also avoid a turbo, missjudge something and you'll be in a world of lag hell and it'll spoil your times.
My suggestion would be a root supercharger, the additional power is always available regardless of revs, there's less issues with high temps because it doesn't rely on hot exhaust gases. I had a Vectra 2.5 V6 with a Walkinshaw supercharger (fitted by Courtney motorsport) and it was the most drivable car I've ever had with forced induction. Loads of torque regardless of what gear I was in, reliable and sounded awesome.