I dont know prices in UK. The only thing I have to say to you is: Dont buy a 2nd hand kit. Only buy everything new. A conversion from a normal aspirated engine to a turbo/supercharged engine involves various calc. If you want to convert a simple engine to a turbo engine, youll have to decrease de compression relation depending on the temperature that your engine can support. You have to make sure that your oil pump support the amount of oil that the turbo will require, youll have to change the oil type depending on the new compression rate and the new engine temperature, youll have to increase the octane level of your fuel to suppress knock, etc
And depending on all this facts youll have more or less power on your engine.