The com pin is only used when inductive loads, like solenoids, are used.
Not needed for LED displays. Leave unconnected, or connect to 12volt.
Connect 12volt to the common anode pin(s) of the displays.
Connect the seven segments via resistors to the seven outputs of the ULN.
Connect seven outputs of the 595 to the seven inputs of the ULN.
Get one digit working on a breadboard before you design a circuit board for four.
Start with 100 ohm resistors, measure the current with the supply you're going to use, correct values if needed.
Google tells me differently. But they seem very expensive.
They should be about US1 each.
5-6 days to deliver shouldn't be a problem.
A clock project like this, with limited knowledge, could be months to complete.
Leo..