Check out Woodmans in the sponsors directory, they are generally the cheapest
But yes £3500 is the average price for having it fitted in my experience.
But many will ask why you want to put this expensive kit on a used car, how long you going to keep it? How many miles have you done? How much warranty do you have etc?
As a GTT kit will be a lot cheaper or you can get more bhp for your money from them
