I'm going to write as a DM for a moment, from the viewpoint of what would I do with players if they began setting up a trading town as described.
To begin with, the hinterlands bordering on this trade town are very important. When the party describes the sort of location they want (empty, with access to other trading towns, on the right sort of coast, at the base of a mountain pass or as a transshipment point on a navigable river), I want to 'sell' them a location the same way a real estate developer would. Yes, things like the movement of goods, production and the political situation are important - but like telling prospective buyers about schools, I always want to throw in that there are nearby hills or mountains, a virgin forest, nearby zone of absolutely no settlement at all and a nearby village where the leadership is corrupt or desperately needed. "Oh yes, great location, lots of nearby borderlands and tribal conflict nearby, good places for finding dungeons and monster lairs, easy access to ruined castles, swamps and abandoned temples. Just the sort of thing for a young party starting out in life!"
If I know my party, I can inveigle them with such enticements, so that while setting up the trading town there are opportunities for a quick jaunt out to the Back Porch of the Flail Snail King before they come back and begin building the tobacco smokehouse. This allows for plenty of distraction amidst the accounting - so that while in their new settlement, they are thinking of the next adventure and while on adventure they are thinking about how long it will take to get the settlement's tavern up and running. As the money runs out, they're thinking of adventuring to get a bit more, and as they find the big pile of gold in the Misplaced Tomb of the Barely Remembered Civil Servant, they're thinking of how this is going to pay for the marble stone floor of their new church floor.
The adventure is thus both a relief and a contribution to the building process. I have found that if I tell a party, "We're going to have an accounting session" every five or six runnings, players are happy to buy goods, discuss plans for something they want to build, ask questions, fill in the blanks on their characters, obtain hirelings, make quick visits to talk to people in town, sketch out future ideas they have with other players and so on. With everyone talking and addressing these issues, four hours can go by every bit as fast as fighting a host of zombies or harpies. Admittedly, I have the kind of players who like to talk about the documentaries they've seen since our last running, rather that what the likely results of the SuperBowl are going to be, but I do feel that players will get invested in things they're making the same way I will get invested in a map or some other design I'm working on.
Where I've left off, the next development of the trading town will be to build interest in what we're doing with peoples farther away than the nearby city. In effect, we've started to build our settlement in the shadow of nearby Athens . . . it's time to do a little exploration into what we might ship in from the islands of the Aegean, the coast of Libya, Italy or the Holy Land. Suppose we've discovered a small silver mine, so we can be sure of having at least one product that will sell overseas - that will let us go out on another adventure, visiting five or six trading towns in Cyprus, Cilicia, Palestine and perhaps even Egypt. All this ship travel will need a ship - so that's going to cost, along with the crew - but it's necessary if we're going to buy things out there that will sell well at home. It will probably be small compared with the immense merchanters we'll encounter.
In the meantime, of course, the ship will get attacked by pirates or sea monsters, there's bound to be some remote island with a good adventure on it, we could get asked to take part in some sort of battle or we could discover a city under seige.
Everywhere we go, we want to be sure to set up an agent who will promote our little settlement, perhaps buy some goods for another journey we could take later (or send a hired captain to obtain for us) and in general spread our names about. It is going to be a great day when a completely strange ship appears next to our trading town, wanting to investigate us and find out how much they can purchase from us!
We've got to be ready for that day. That means being goods-poor for a time, steadily piling up more than just bars of silver or the best timber (spars for ships, perhaps, or some unusual wood for furniture/instrument making abroad) that will sit and sit and sit, waiting for the day when that buyer shows up. All that wealth is bound to encourage someone to try to plunder it - so that's an adventure too, as we test out the quality of our own defenses against an incursion by raiders from land or sea.
There are many opportunities for the players to still make experience and go up levels. We shouldn't overlook that.