Money-making tips:
If you start as a faction with positive beginning balance (Romani especially), you're basically set. No debt means you have solved the biggest early game challenge by default. To stay that way, judiciously pick your wars and armies. Your starting forces, when combined, should be well sufficient to the task at hand of driving Epeiros out and beating on the tough Eleutheroi nuts above you. If you're one of the other big powers with no debt, the challenges are different. Unless you're Carthage. They must be the easiest campaign of all: no enemies around you until later, great economy and expansion opportunities, and a great unit roster. But the AS/Ptolies have each other to deal with and the AS have the added challenge of a rebellious and dangerous eastern front to deal with.
In any case, focus on economical improvements: roads, markets, ports, and mines. While mines should be built as soon as possible, they are pricey so early on you may want to focus on the infrastructure. The markets and ports especially can really boost your income for minimal investment. When conquering settlements early on, try and avoid sacking them as the larger populations = more money and controlling them should not be a big issue until they hit Large City or more. Of course, do note that tax income is not a straight line but more of an S-curve (or maybe it was logarithmic...I forget), so when you get to the point where you are conquering those big, rich cities, sack away, as they will regrow swiftly and you will get truly kingly sums of money from sacking.
As for armies, try to stick to mid-range units until you are solidly into the mid- to late-game, as they are easily your best bang for the buck. The Romani are an exception as they lack a truly "elite" unit but their unit roster form top to bottom is very solid, so you can build a full consular army of roughly historical ratios and you will get great performance at relatively bargain prices. Once you hit the Marians, that's even moreso as you get a great line infantry that comes in large bunches (202 at Huge) that is only relatively weak against heavy cavalry charges.
Pick your wars carefully. If you let yourself get dragged into multiple wars you will end with no trading partners and lots of armies to pay for. Even though the AI is homicidal and suicidal usually in diplomacy, they can still be played against each other if done right, thus keeping them just busy enough to not want to fight you. Posting your largest garrisons on AI borders is also a nice deterrent, as the AI loves to attack if it sees an "opportunity".
Now, if you are not one of the lucky factions, you find yourself in the red, often cripplingly so. As has been repeatedly stated in many threads, you have two choices: expand, or disband. Usually expansion is your best choice, though if you have cavalry you can usually disband that and save valuable mnai in upkeep. Also, get those taxes raised as much as possible to stave off the inevitable. You are almost guaranteed to fall into debt, sometimes a very significant amount. Just keep conquering as much as you can (using guard mode judiciously can really help) and you'll see the red start to slow down, go down, then disappear. For example, with the Arveni, I was able to beat the Aedui, leaving them only with Mediolanum (I wanted them there to tangle with the Romans), and conquer all of Gaul except Burdigala and Tolossa before I had basically exhausted my line infantry, so I disbanded my cavalry finally (yes I kept them all and they were crucial to my wins over the Aedui). I maxed out at roughly 23k of debt but after disbanding my cavalry when I was done in my initial wave of conquering I had an income of roughly 4k per turn and my debt had already gone down to 17k, so by 267 BCE I was in the black full speed.
Anyways, hope its all useful to you. And my lone input in the little "how realistic is the building tree blah blah blah": I view the player more as the Guiding Hand of Fate or some other deific force rather than any actual person in the game saying "build this". Thus what goes on is more an abstraction of all the various interaction, decisions, politics, and pragmatism that go on that cannot be represented in a war game, necessitating the usage of constructible objects to represent such things and the boni or mali that such decisions et al may confer.