Indestructible:
Well for starters it doesnt need to be repaired. Which when your wandering the map isnt a problem. But every dungeon I have hit, the limiting factor is usually my crossbows burning out and needing repair so I head back. The lowest of those is 80 durability.
The most important thing though is upgradability.
When you use the blacksmith the weapons can take a hit to durability each time it is upgraded. Sooner or later you have to stop, since durability is going too low, useability and lifetime becomes a problem. Its not a big problem at first and doesnt have to be a problem at all.... since 3-4 upgrades can give you some nice ~20dmg bows, thats plenty good for wandering the map and dungeons up to 2-3 also. If you want to keep focusing your money into pushing one party to power though, you need a way to get more upgrades on those weapons and armour. Indestructible items are limited by cost only. Maybe some end cap eventually.
It may be that pushing upgrades so high isnt required at all, but its likely that the other players (the long term ones) have at least pushed one party to a very high level of upgrades, if not a good few. We need a credible threat to match them and to gain respect for the dwarfs to negotiate on any level of the field, be that war or diplomacy. Having a bunch of highly upgraded warbands in the hands of more players will be a good first step.
Enchanted items, are found in dungeons and also in high prosperity cities. Whether the enchanted items in cities are just there because people sold them there, I dont know - it is possible we are just seeing sold loot from the other players. Dont be shy to explore Staberinde cities or the orange guys. Yea the taxes help them out but imo the cost is minor compared to what they probably make in taxes all over their empires. A few weeks of you paying taxes to those guys is nothing if you can grab some good loot to accelerate you.