I use quick yeast, it comes in little 10g sachets, and claims to be enough for 500g-1kg flour. I have used it for varying quantities, sometimes as much as 1.5kg flour, and it has always worked for me.
You simply sprinkle the yeast onto the flour, but you must be careful to mix in any sugar or salt prior to adding the yeast, as these can affect the yeast's properties. It has vitamin C added, which makes it work better.
I have seen this product in the "home baking" section of Sainsbury in the UK. The beauty of it is that it has a good shelf life. I don't know about bread "improvers"; my feeling is why add unknown ingredients to a good, natural product?
I always make my own bread and rolls; I have a B&B, and my guests appreciate the home made goodies.
I gave up following recipes when I found out just how easy bread is; try using half white and half wholemeal, and adding sunflower, pumpkin and linseed seeds.
Also, adding half a cup of semolina and half a cup of polenta to white flour.
I also add rye flour, about 1/3 to 2.4 white flour.
Good luck with the bread. 