Re: Building strategy as the Nervii?
Have you seen the Nervii building chain here?
You mentioned keeping food and public order under control. I haven't played much as the Nervii - someone else might be able to offer better advice - but I have played as the Iceni who have similar buildings.
For food, I rely on fishing ports and farms, as you would probably expect. If you are used to previous Total War games (before Shogun II) you might expect to upgrade everything to its highest level. However, that will cost a lot of food or public order (particularly for higher-tier buildings) - it's usually necessary to choose carefully which buildings to upgrade and when. It's worth bearing in mind that food is shared across your empire - it doesn't matter if one province has -10 food, if your empire as a whole has +20. (Unless another faction invades the regions producing your food, of course.)
For public order, I like the Horn Maker line of buildings (Artisan's Lodging, then Horn Maker). They improve public order, provide money and add garrison units.
You mentioned the problem of affording high-tier units. If you can do so, I suggest specialising one or more provinces to food production and producing wealth. As a barbarian faction, you might want to consider creating trade partners by occasionally using the 'liberate' option when you capture cities. In my experience, liberated factions are usually willing to trade and can provide a useful defensive buffer against enemies.
You mentioned going military as the barbarians. That makes sense; you might want to experiment more with using low-tier and middle-tier units, since they are cheaper. If you haven't already found them I recommend the Rome II tournament commentary videos by Maximus Decimus Meridius (on YouTube) as he provides useful tips on making the most of units. Even humble Levy Freemen can play a useful role, absorbing enemy charges and using their spears and precursor javelins to provide a defensive screen against enemy cavalry.
(I'm sorry if I'm telling you things that you know).