I haven’t played in a few years so take this with a grain of salt.

My approach is to build things out incrementally. If your cashflow stays positive the entire time, you won’t have money problems.

For design, try to find modular chunks you can repeat. A cellblock with bathroom and showers. A canteen. Make blueprints from them and twist them.

Do you have a foundation and walls around the entire space? I usually kept most stuff outside with a fence around the perimeter.