Came in the SS2 only I believe. Pretty rare, grab one if it comes up. Your looking for the vin code with it on if your looking at cars. Only made for a couple of years.
Price varies, figure between 150 - 250 with drive shafts. Drive shafts on here own if the person knows what they have seem to be around 100. Also can be feckin hard to find!
I didn't fit it (assume Bek has chatted to you) but my gear cables aren't quite correct size, grab the gear cables with it, or I believe (well, I going to grab some!) GT4 St205 gear cables will fit.
GT4 clutches fit it, if it doesn't have a clutch may as well get a 3sgte one and pressure plate.
Believe one gearbox mount needs to be sorted as there is one different to your s54 - hopefully someone wiser will know more.
There is a st185 gear box modified for fwd on eBay as well which might be worth a thought (or was if its gone!). Not sure it has the right drive shafts though. I believe the e56 is pretty much the gt4 box without the transfer bit.
Oh and the 3sgte is brilliant fun, you'll love it! Am in southampton fairly often so if you want to check out mine first, let me know.