Invest in Fletching, or just keep them stocked. Archers are REALLY strong, because they can attack twice a round with the same weapon (as opposed to shield bash, the only other method) and that makes them super dangerous. They're a bit more costly to keep around, but they're also pretty awesome.
One way you can offset the cost, is to put any type of person on garrison, perhaps a wizard because high Int, wielding a bow with great Fletching, but on defensive. They won't use the arrows, but they'll produce them into their own inventory. Then you can come by every few days and collect the gold and arrows.
As for other weapons, the Battle Axe is pretty great. It has the highest Damage and Attack of any 2h, except the Halberd. The spear is a bit underwhelming, kinda lacks attack power, but has good back-row attack, so it'd be good for a secondary warrior. And it is indestructible, so it has an innate advantage. The short sword, well, it's indestructible...
Cost for upgrades are something like 200 - 800 - 1,600 - 5,000 - 21,000 - 52,000 at least as far as I've gotten, still saving up for that 52k upgrade... It gets really expensive really fast. Thankfully we have some godlike blacksmiths, so you at least shouldn't waste money on fails or sub-par upgrades.
Also, for movement, I usually give them rings.