If you're drawing heavily on the Hellenistic era, Paul Elliot's Warlords of Alexander is a useful source to draw upon.
That's not really true even of the Classical period in the east or the steppe. Composite bows are common there, it's only the west that had underpowered self bows.
Slings are certainly popular in the west, and with a cast bullet, rather than pebble or stone, can outrange western bows.
Again, not really true of the Hellenistic era being drawn upon. While the hoplite panoply is the heavy standard, there's still scale and lamellar thoraxes, as well as textile armour for the torso. Any of which can be supplemented with greaves. Add a helmet and shield (almost everyone had these two) and you're set.
Note the shield is the most important defensive item in this period, not armour. With a helmet and body shield, you're pretty well covered as well as being mobile.
Mail is common among Celtic nobles and beginning to appear more widely. You're right that mass-produced mail didn't appear til later in the Roman era.
True, as are javelins. Lots of line infantry had javelins to throw before melee contact - something inspired by the Celts who used it very effectively.
Once more, not true of the Hellenistic era. Once the treasuries of Persia had been looted, ther was a lot more coinage around. Gold devalued from being worth 27 times as much as silver to 10 times as much, because there was so much of it washing around. Persian money (especially gold darics) were a common way for mercenaries to be paid, and the Greek currency (silver owls, etc) were close to a common currency. Along with bars of silver/gold for trading volumes of transaction. Though many more advanced states would have used a credit system to spare the risk of pirates carrying off your wealth.