STI is stronger engine (Not much but it is!), different map, bigger intercooler, different suspension etc. etc.
How much power do you want?
Put the turbo engine into your car is entirely possible, but you need to change all the wiring looms and the gearbox. Apart from that easy job I think. Time consuming though.
£15k will get you a MINT STI6 Type R.
I think if you made Aircrest and offer on his STI5 Type R though he would do it. Manx registered etc. Look in the Manx Directory for Aircrest Air Conditioning etc. or emailling aircrest@manx.net