You are really talking about 2 topics:
1) The advantages of an external HD.
2) Backup/recovery procedures.
1) Adding an external HD may/may not improve performance. It will certainly improve your available capacity. You havent told us how this HD is connected or what the technology is, but the critical performance criteria for disks are spin speed (rpm) and the connection technology. External HD are typically SATA drives with a slow spin speed usually 5400 or 7200 and connected via USB. USB is rated at 480Mbits/s whereas SATA is 3Gbits/s and the USB hardware is not typically optimised for disk performance (ie its nearly always a shared channel)
2) If you had a fire recently i hope you are seriously evaluting your backup/restore procedures as an external disk isnt the best option here. ie if you are considering unplugging the drive everynight and putting in a firesafe or taking it offsite, you need to consider how many insertion events the connector is rated for (its down to the metal (Gold) thickness on the connector), its usually only a couple of hundred, so it may last a year.
I think professional help would be a good idea.
If you are referring to the Installer (Main installation package of the software) located in your Drive C, you can easily copy those installers by pressing Ctrl + C. However, if you are referring to the programs already installed in the drive C, it is only possible if you will use a disk imaging software like Acronis True Image.