A lot.
You're probably looking at £3.5 - £4k to do it really well (remember, you also need better brakes, wheels, suspension, tyres).
But cost it up.
Standalone (cheapest is DIY Megasquirt)
Rolling Road time
Turbo
2" exhaust, bigger CAT
Exhaust manifold
Fuel Pump
Injectors x 4
Inlet manifold
Intercooler
Oil Cooler
Turbo
Forged Pistons
Forged Rods
It's probably going to be cheaper to pinch the 1.4 16v turbo engine (T Jet) and box out of a 500 Abarth. More power, better reliability, tougher gearbox.