This scenario stars off with France fighting other powers, but not Prussia. Therefore, the biggest choice the Prussian player faces at the outset is whether to ally immediately with, say, Austria (which France always attacks), or perhaps one of the other countries, and start fighting France right away. Since France is hard to beat, and one doesn't win COG:EE in the first few moves, my preference is to stay out of it and work at strengthening myself. After all, there will be plenty of opportunities to fight, so I'd rather do it after I'm wealthier and stronger.
One crucial step towards gaining strength is to create another army for all those divisions and generals who are just standing around. So, even though there are several good options for spending my money and resources, I believe this to be the most important. After all, I might have no intention of attacking, but someone might attack me -- and I'll need a second army in case that happens.