My first problem is not to go into debt. I have to import iron and wool to make weapons and armor, and the export trade routes are a bit expensive. My second problem is to be able to fulfil those initial requests from Rome for weapons and armor and still be able to deal with the Gauls (buying them off causes me to go into debt and raising a military conflicts with Caesar's demands).
So far I've settled on starting up the timber and furniture trade (I need these two for the iron and wool anyway) and making all my own food and goods (meaning that I have to make do with one basic (oil) and one luxury (furniture) for quite a while). I can't afford to import pottery. I plugged down one weapon and one armor factory and recruitment post (along with a mess hall and heavy infantry fort) and micromanaged the imports (to make sure I didn't import too much) until I started making money. I still went into debt a couple of times, but not much. But I couldn't satisfy the demands from Caesar as I needed the weapons and armor myself. The good thing about this is that I was able to beat the first Gaul invasion and that I'm making money now so that I can slowly expand my city and work towards the prosperity goal. If I have to, I can buy myself out of the favor goal - that's in fact way too easy to do.
Is there a way to make money, satisfy Caesar and beat the Gauls at the same time on Hard?