Just thinking, instead of purchasing a brothel make all buildings in the game non-specific. Instead list the number of large rooms, small rooms and any special traits this location has. Buildings may be offered for rent, purchase or both. Rent should be about 2-4% of the purchase price to make renting viable. In either case there should be a maintenance charge associated with the use of the building which includes repairs.
Before traits are created you would need to work on the idea of a business. What I am imagining is that a business is an entity that is created on a building site and could move if you have an empty building (or switch with another business you own). Each business would have a name, and stats. There would be a list of employees which is limited by the building it is located in. A certain number of rooms would need to be remodeled to match the business. For instance every brothel needs a parlor and bedrooms. Every armory needs forges, smelters and at least two store rooms. There should be a list of necessary and optional rooms for each business which would list the size requirements of the room, the cost to remodel the room, and what benefit gets applied. Among the benefits should be listed how many employees you can add. The business would get stats based on a percentage of the employees stats (something between 1-10% of each employee). For instance a brothel might have charisma, reputation, and various sex acts as stats. The number of customers would be affected by the reputation of the business. Each individual girl would check to see how they perform, but could receive a bonus (or penalty) from the stats of the brothel. Say something along the lines of the brothel's matching stat minus the customer's expectations. So the higher class of customer the harder to meet their base expectations (with matching increased payment) but the larger bonus for a well established business. The percentage of how much each employee contributes could be a sliding scale, starting at 1% and going up each week they work there till the reach the max of 10%.
Every business should have reputation, but other stats would be based on the business. For instance our armory could have stats like reputation, con, strength and smithing. The number and quality of customers is based on reputation (plus smithing for quality). Each job would require an individual girl to make checks on con and strength to make sure the work gets done on time and isn't accidentally ruined (should be an easy check with the shop adding a significant bonus. Maybe x2 or x3 the flat stat?). Then check against smithing to determine the quality of work and final payment. Say smithing + shop bonus - customer expectation level. Test gives a sliding scale of payment starting with no pay (fail by over 20?), half pay (fail by less than 20), normal pay (success), 20% bonus pay (success by over 40), to 50% bonus pay (success by over 80). Again as the quality of customer goes up the base pay increases, but so does the expectation level. Someone ordering clubs isn't expecting high quality work but even with a masterpiece they aren't paying a whole lot. Someone ordering a full set of maximillian plate armor is going to expect master level work and is going to pay a small fortune for the best.
After determining what rooms each business needs you can offer to place them in all buildings with an appropriately sized room, plus some utility rooms like kitchens which reduce the maintence cost for the girls that live in that business (eating in instead of buying meals). You'll also need to determine if employees need to live on site or if your going to establish some sort of apartment system where girls can live in one location and work in another. I would raise the maintence cost or amount of fatigue if they didn't live on site.
Traits could be things like Lavish (bonus to charisma and quality of customer), High Traffic (bonus to number of customers), Low Traffic (penalty to number of customers), Solid (difficult to damage, reduces maintence cost of business), Garden (-fatigue, bonus to certain skills), View (+reputation, extra bonuses for a few business like restraunts). Some traits could be added or improved. For instance Lavish could be added or improved at a substantial cost. Traffic on the other hand, can't be improved. That would be a reason to move the business.