I think that there was a lot of glossing over the question of the DM in regards to spending wealth.
For example, "Buy a fort" sounds great... but where are you building it? Build it near a city or a town, and you're going to find a very annoyed lord asking who the hell is building a military fortification near his town. So now, adventure hook right? Convince the lord to let mercenaries build fortifications near his home... except being even the least bit realistic about it means taking weeks to negotiate contracts for defense and interviewing people you are hiring and all that.
And frankly, none of my players have ever been interested enough to even want to buy a house. They don't care about that stuff, because they care about the plot of the story which has them moving around rather constantly.
And, that "hire a mercenary army", why the heck would I want my players to do that? It's going to work one of two ways. Either it will be a footnote, "oh your army killed that orc encampment three days ago, there was feast, and morale is up" or they will bring them with them on the adventure. I've got enough on my plate running the enemies without having to also run the 50 soldiers following the group around, which if they participate in combat will slow things down horrendously. Plus, why the heck were their 50 unemployed soldiers wandering about the town? That's kind of a big number, equal to the watch maybe? That is a serious civil problem.
I may sound like I'm trying to find problems with these ideas, but from my perspective as a DM, if the players aren't interested in the political intrigue that comes with trying to outbid nobles for land and people, then I don't even want to get started with them simply buying their way to success. It simply is poor for the storytelling aspect of the game, if the story players want is to be fantasy heroes who go out and do things, instead of landed nobles who buy spies to run counter-intelligence on spies who are trying to disrupt your grain caravan.
They can both be fun, but my players don't want a bureaucracy so they really aren't going to find much use in getting "realistic" with spending their money.