Depends how much power u want in the end. I'd go for a T3/T4 garrett turbo, front mount intercooler, bigger fuel system & if the engine's done a few km's rebuild it with ACL pistons & rings with a lower compression ratio of about 8.0:1. It should easily give u around 400hp with mild boost. You'll need either a new computer or a unichip piggy back aswell.
I got a quote from GCG Turbochargers in Sydney for $8500 for everything.
AVO in vic can do it also for a round the same money.
Theres also a guy on Fullboost.com.au who has started to make kits for the 4.0L six. Here's a link to the thread http://pub23.ezboard.com/ffullboostf...icID=921.topic
might be worth contacting him for a quote. His user name is PSI060. His getting about 500hp.
Also check out Zoom magazine. They did a twin turbo setup in a XR6 ute, using second hand nissan turbos. The project was called "workhorse wake up". It went over 3 or 4 issuses.