your best bet would be to pickup a GQ with the TD42 in it, take what parts you need
engine / fuel lines / wiring harness
flog the rest off
buy a 2nd hand fuel tank from a wrecker
give yourself a couple of months (every night & weekends) and you can do it
as stated, a mechanic will take 2 weeks if your paying top dollar
2nd hand GQ with all parts needed (bonus if it has diff locks & other bits you want) - around $10K
someone to strip the parts out you want - $2500 labour
mechanic to remove / refit engine - $2500-3500 labour
new Turbo & Intercooler - $5000
2nd hand engine with kit from wreckers - $7000-13000
mechanic to remove & refit - $2500-3500
not including incidentals - water pump / fan / fan hub / radiator shroud / radiator (maybe) fuel lines / new oils / new coolant / etc
now, you have not said what your like with a spanner, so I am assuming as you suggest a Mechanic, you dont feel comfortable with doing it yourself?
there is some places that import stuff from Japan, you may want to ask them on a price for complete TD42Ti - just to give you an idea
also ring around the wreckers to find a GU TD42 (T or TI) and see what price they are charging for the complete car V's just motor and parts you need
keep an eye out on the Auctions for one "cheap" for parts...........