1. Different sources have different answers, ranging from 2:1 to 4:1 fresh:dry. Fleischman's and the Food Lover's Companion say that a .6 oz. cube of fresh is the same as a .25 oz. packet of active dry or instant yeast. This means a ration of 2.4:1, so divide the oz. of fresh by 2.4. 2kg/2.4= 8 1/3 kg of active dry or instant yeast. The type of yeast you use and whether you use a little more or less, as well as temperature will affect your proof time.
2. I don't know. Maybe degrees means pH? I've never really measured how acidic anything is that I've baked before.
3. Short answer: it'll be okay to substitute fresh milk. Longer answer: depending on the amount you substitute and whether it is whole or nonfat, you may affect the amount of protein in the recipe, which will affect gluten development somewhat. Will it make a huge difference? Probably not.
4. Butter should be fine.
5. According to the German numbering system, 550 is AP white flour and 812 is white bread flour. According to Uncle Phaedrus, there is a Swiss number system where 600 would be white bread flour, and 800-850 would be whole wheat flour. I haven't found anything else mentioning this numbering system, so I'm kind of dubious of that answer. So really, I don't know. It looks like you're making some type of regular leavened bread, so I would guess you should go with white bread flour for better gluten development.
I would definately recommend following the procedure in the recipe closely for your first test. The first step is called the sponge. It is important because the initial fermenting period will impart flavor to the dough. The longer it ferments in the first stage, the more complex and sour the flavor will be. Also, it allows the flour to fully soak up water and autolyze. Dough that is autolyzed will develop more gluten faster when it is kneaded than dough which is not autolyzed. If you are only kneading the dough for 5-8 minutes then letting that dough sit is going to make a difference in the end product.
I suggest you try the original recipe, as close as you can get to it. Then make it again a bunch of times, tweaking things like the amount of hydration, amount of yeast, temperature of preferment, time of preferment, and powdered vs. fresh milk. I also highly recommend checking out thefreshloaf.com.