It is not possible to use an external hard drive nor an USB disk. There is no USB connexion (except the one used by the ARM controller to be programmed).
When you receive your MiniMig, do not hesitate to use an 8 GB card (the ARM controller allows the use of bigger SD cards) to replace the 2GB one.
When formatted in Fat32, you'll be able to create 2 virtuel hard drives (3,9 GB max each). It will be more than enough for an Amiga 500 and will store a lot of WHDLoad games.
With the MiniMig you'll receive (the config you are talking about), you won't need anything else to start having fun with it