I would really like to be able to construct how a perfect guild hall would be in my view before i'd actually do it in game, because i'd need to cycle through a lot of choices. I'd have to have an astronomical amount of crowns to do it in game, so i want to ask if there is something similar to SpiralSpy which would allow me to see an entire guild hall, assign rooms, add furniture, etc. If there is none, how hard would it be to make one?
There isn't anything like that. Making it would probably take a whole lot of effort if you really wanna emulate Spiral Knights' mechanics in terms of the grid and whatnot. If you know programming, you could start by looking at their GitHub repositories. If not, your best bet is probably trying to infiltrate a rich guild and use their Guild Hall ;D