The configuration of everything is in config.xml including package settings.
The trickier part of really being "ready to go" with the backup system is that you need to get the actual package code/binaries onto it. If it happens to use DHCP on WAN then you can plug it in somewhere that is not the live office LAN (because that IP range will already be on the LAN side of the backup device) and let it get DHCP. Then you can do an upgrade of pfSense to the latest version and let it install all the packages… while it is running a copy of your real config.xml.
If the production WAN settings are some static IP or PPPoE or... then it only works when connected to your production ISP link. You have to either:
a) Modify the WAN settings to get it internet access from somewhere, do the upgrade, package installs, change the WAN settings back to (hopefully) the correct ones for production, or;
b) Take the production system offline for a bit (downtime), put the backup system in place, upgrade the backup with package installs..., shutdown the backup spare and put the production back online.
It is all a bit tricky to get a full operative cold spare installed and completely ready-to-go in a reliable way without interrupting production.
Maybe someone else has a good method for this?