The Stillen kit that you linked will just the basic one without intercooler etc... The general consensus is that that will give you around 350bhp. To get more out of it (if this wasn't enough) would cost you more pennies, in which case the HKS or Vortech kit would be a good choice for around 400bhp.
If it were me, I would buy one of the second hand kits for sale on here as they will be ready to go ( minus the tune) and will save you alot of £. If after you realise its not for you, you can sell it on, on here.
I know how you feel about the car, I have loved the look of the 350z for a long time but every time I drove one I was under-whelmed.......until I drove a supercharged one. In my opinion it should have come out the factory with similar power. I love it!