I'm happy with you drawing up a plan on what you want and giving you an estimate on that.
From my perspective, Inns, merchant's buildings, etc. are probably upper-middle class affairs. A farmer would not be able to afford one, as a farmer probably lives on a gross income of a few gold a year. A shop or innkeep would likely have a couple hundred gold to may be a thousand gold a year, depending on location, etc., and I'm guessing living on 50-100gp net, which still puts them in an upper-middle class status. Shops like this would likely be passed down generation to generation, allowing the younger generations to make more money because the cost of the structure is deferred. Thus, for someone like Sterling with thousands of gold in disposable income, and more if the other party members are willing to go in as partners on it, building an entirely new structure for even 10,000gp (which would be a complete rebuild and probably rather nice), is quite doable. Buying an old structure for 5000gp would be easy. The Isles are going to give you a decent price because the population has dwindled lately, and the land has been devalued through occupation and a civil war. An inn like this in Torander's Bluff might be twice the price or more, sort of like going to New York or London.
Given that Sterling grew up largely in Anorien, I would suggest that his values are mostly derived from the High Elven culture there. The High Elves typically wait and have long courtships before marriage, due to these being centuries long commitments. Marriage is semi-formal with the High Elves. They don't have all the formalities of the Grey Elves, but neither do they have the complete freedom of Wild Elves (who sometimes simply say a few words to one another and declare themselves married!). The ceremony isn't that far removed from modern day human ceremonies, where you have a large latitude of what you would like, but vows are typically said in front of friends and family, typically in the name of one of the elven gods. Dowries are usually not given, and marriages are usually not arranged (except in the case of royalty, etc.). The names of the elves remain, and the children usually take the last name of the father.
Seldar is more formal and traditional medieval. I mention Seldar because that is where Arienne's father is from. Dowries are usually exchanged, and the marriage ceremony is much more patrilineal, with the woman taking the man's name and the children taking the name of the father as well. That being said, Arienne's father married an elf, so his take may be unusual for a Seldarian man. I think there is definitely some room for flexibility here.
Customs are all over the place for elves (so you can make up what you want), but typically in Seldar a quality dairy cow is given by the male to the female's family a a sign of his desire for betrothal.
Let's go do whatever was the last thing that Morlan thought was a bad idea.