Im sure you could do it for £4k, but you would be searching for the right parts cheap. As I said, my Rotrex sold for £2500, and it was 5 months old with less than 8k miles on it. They very rarely go up for sale.
The turbo in my opinion is much better, with the SC you only make peak power right at the top end, the faster the engine turns, the more power you get, so if your car makes 350bhp peak power, you will only have that power for a second before you need to change gear. The turbo makes more power quicker, so you will have peak power for longer in the rev range. Also if you get bored of the power and want more, its more difficult with a supercharger, and you only have 1 or 2 options, which is a change of pulley. The Turbo is just a retune and up it, or you can buy a bigger turbo and go for monster power.
In my opinion, the turbo is easier to handle, better fuel economy and sounds 100% better. Ive bought and sold a supercharger and replaced it with a turbo, and there are quite a few FN2 owners who have done the same.
Have a read through my build thread its full of useful information. But its all about what you want from the car at the end of the day.