Nik you won't go wrong with this one. I bought the EXACT same turbo off eBay from ApexBoost a seller in Ventura, CA he offered to polish it for free and machine down the 4.21" to 4". I offered $750 and won it.
http://cgi.ebay.com/ebaymotors/Borg-Warner-Turbo-S300-S366-Turbocharger-Extended-tip-_W0QQcmdZViewItemQQhashZitem2c5d9debbfQQitemZ190549191615QQptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories
My mechanic has a similar one and loves it. He actually told me to get this exact one 2 years ago and I really wish I had. He is infact looking at getting one since he says it's actually a better one than his due to the something or other being twice the thickness. Sorry but I'm not a techno. I went with the GT35R instead since two buddies told me that that was the best street turbo...blah blah blah. Well the GT35R only lasted not even two years and it can't be rebuilt so that's $2100 wasted when I could have gotten this one for $775 shipped. It spools fairly quickly, has a lower power band which is good for street and track so it's not laggy. I'm running it as a single on my 89 Supra Turbo with 2JZ-GTE. My goal is only 750 RWHP not 800 so it will be fine. Not sure how it will work on a Skyline but it should be awesome if it's anything like on the Supra. If you can get it shipped to Oz from Cali then you have a winner unless you can pick one up there. My mechanic's buddy has one in his Nissan 300Z and paid $1000 so I figure I made off like a bandit. Cheers.