I bought a low milage 20G for £640. when I was looking to fit a 2.5 Bottom end. New they are Just under the £1000 mark. (See Scoobyshop).
A TD05 18G, converted to Front entry will be around £400, second hand for a good one.
The trouble with a new Turbo is, that other things will need to be uprated.
If I remember rightly, your car is based on the WRX?, so you will need a set of STI Injectors and perhaps a Walbro pump.
That's approximately another £300.00.
Still in Budget, if you fit yourself.