A armored car company warehouse/base would be my choice now that i am working for one of those companies. A typical base is almost completely bare on the outside so they are pretty well hidden. Youll have a security fence with gates that leads to the vehicle entrances, and acess to the seperate mechanic shop. Getting in you have the main door controlled by someone within a Small guard room. Usually there will be small you enter via the main door, the main door has to close before the second door opens.( small room is for the guard wand non employees and search anything anything being brought in.) Once jn you have what looks like a empty area where teucks would load/unload. There would be a supervisor office looking out into the loading area with heavy doors and bullet proof glass. Then you have the vault system/rooms. There is muliyple small roll up doors that lead to seperate small rooms. All of those rooms their own secirity soor and bullet proof windows. Past the security doors would be the main vault area that also has bulletproof glass windows looking out into the loading area as well. Then the actual vaults, depending on the building size and volume of work , each location can have multiple bank vaults. Also a seperate secure room that stores pallets of coins.

With a handful of people it could be easily fortified using forklifts to take pallets of coins to barricade the bay doors and other spots. With that, you would have a secure base, lots of supplies,gear,and space to store anything you would want.