8oz of castor sugar is about right; if you opt for a demera sugar try 10oz.
That's way way too much sugar unless you really want sickly sweet plonk. The berries are not that hard and only take a few weeks for the juices to leak out.
As for leaving it 2 years, I don't know as mine usually just about make it to Christmas. I'm now looking at 2 lovely rosy red bottles that are at least three weeks old and wondering if they ought to be tested or not.
All I do is put sloes in an empty spirit bottle saved for the occasion, with berries coming about a quarter way up, cover with sugar (3-4oz) and shake daily for a few days until sugar is gone. The only hard part is not opening it.