Quantity   Component name
1 × 3D print of the mini-breadboard support rails (optional)
1 × 32k AT24c32 I2C EEprom module (optional)
1 × Wires, resistors, leds, holders, connectors, etc ( $3-10 ) I splash out now on soft silicone wire, and pricey Deans connectors, but early builds used the scraps I had lying around and they worked fine.
2 × 25-TiePoint mini breadboard
1 × Any Pro-Mini style 3.3v Arduino ( $2-$15 ) i'm currently using Rocket ultra boards, but have working builds with many others including some fairly dubious no-name clones. Note that you must use a 3.3 volt Arduino to connect the SD card directly to the pins! Also note that the pin layouts vary from one board to the next so you will have move some jumpers to accommodate that.
1 × DS3231 & 4K I2C eeprom RTC module ( $2 ) I prefer the DS3231 over the DS1307's because they have much less drift over time. But both work with the same libraries.