Sachs, Valeo or go for the whole RS4 bits and bobs. I'm not a fan of single mass due to some horror stories but at that kind of power it's a requirement really, just out of interest what do you have in the pipeline to produce this power? Would personally go for the clutch after performance mods are sorted.
Valeo probably has the best rep at the most affordable price, labour will be a fair bit too unless your sorting it yourself. I'm not very familiar with petrol's but would imagine you'd also need different injectors, 4-5 bar, 2.5"-3" exhaust and different fuel pump for 300bhp.