I think you’ve covered the key components — microprocessor, WiFi, solenoid drivers (i.e. relays). There are other comopnents that OpenSprinkler has, but you may or may not need them, such as the 24V AC to 5V DC switching regulator (you dont’ need it if you are ok with using two separate power supplies, one for the microprocessor and one for sprinkler valves), real-time clock, buttons, LCD, microSD slot.