check the developers guide (linked on the resources of the product page - direct link). You only need 3v3 for operation. 5v power on USB connection will come from PC, but is not used to power the board (see developers guide) and therefore a 5v supply will not be necessary to debug your app.
You need LDR’s for booting into bootloader mode so you can update firmware. Check the G120 SOM datasheet for details. If you have the full GHI loader installed you may not have ever booted into bootloader mode yourself but let Fez Config do it for you, but you should still know where the pads are in case you ever needed to do that manually to erase a rampant application or recover from some other situation.