I was in the same position 4 years ago when I looked to increase the power of my 2.8 V6 Sierra.
The cost of the Cosworth V6 really is expensive to obtain in NZ. There was one advertised on TradeMe, (I think last year) it came with Jerveys IIRC went for over $5000nz.
Plus to get to 300hp would have needed quite a bit doing to it.
I think bang for buck and ez of installation a 4.0L OHV would be worth a look. I don't think it would take too much to get 300hp out of one of these. Parts seem quite a bit cheaper
coming from the US. Or though, with our NZ$ crashing against the US and the UK Pound at the moment, nothing is going to be cheap.
If money isn't too much of a problem then a 3.7L Cyclone V6 engine would be the dogs nutz.