As to why the Dwarves in DF should want to use money... assuming the economy is turned on, and they are allowed to buy jewelry, clothes, and personal items from other dwarves, visitors, merchants and the fortress itself, money can fill in the gaps of barter trading (if a dwarf's spare craft they are bartering away is worth 120 and they want something that costs 160, they fill in the gap with a few coins, with some variable amount of coins between 0-40 traded based on the bartering and appraisal skill of the dwarf they are trading with). To incentive the player to introduce money, the dwarves could get positive thoughts from trades that are nearly even on value (thanks to coins smoothing out the difference), and negative thoughts on heavily skewed trades or trades that are stopped because they are too uneven and their is nothing to facilitate the exchange (because the player failed to make money).
I think the steps I propose could encompass multiple types of player fortresses (communist fortresses with a high UBI and workshops not put up for sale/rent or the economy kept only at partial activation. Capitalistic Fortresses with all of the rooms and workshops going on sale. Feudalistic fortresses (by editing the raws to grant nobles rights over workshops and farm plots), where the means of production are owned by nobles who rent them out and make money from an underclass that way). The key from a play standpoint is making sure the player is rewarded/incentived and that basic supply/demand rules are used to keep prices and costs in a reasonable range for the dwarves.
Good suggestions about the transition between primitive communism and pre-modern capitalism. I could even see mixture between these regimes, like with some industries under total control of the fortress and freer ones.
One of the motives of migration might be the desire of the settlers to become rich; to modelize this, once the Economy is enacted, part of the property of the fortress, excluding items such as weapons, prisons, libraries and other related to public order and government, shall be given to the first seven settlers (for exemple 40%), with each of them receiving a share, with the leader receiving the double. This way, they could become a sort of nobility, or
patricians.
Small nitpick: feudalism would have involved the privatization of public powers and the patrimonialisation of public law (fiefs were eessentially territories whose owner was also the ruler and could levy taxes and have militias; in the Holy German Empire, jurisdictions were bale to be sold and bought); I could see some of the deeper caverns having large autonomy under the rule of their owners.
Anyway, I posted some ideas about the modelisation on the thread
Torts, Contracts and Marriages, or Civil Law in Dwarf Fortress.