Had a chance to start getting supplies to begin the prototyping and basic code work. Everything won't get here until late December - mid January, and it's not all the parts, but it's enough to start learning the Arduino IDE and creating all the software. Only totaled out to about $30, and with a couple of wire swaps depending on configuration, I can test up to four soil sensors, and the LEDs on the relay board will simulate the solenoids and light connection. Got everything off of eBay China, and here's the list of what I got:

• Arduino UNO R3: http://www.ebay.com/itm/111705016306
• Data Logger shield: http://www.ebay.com/itm/131176003874
• ESP-01 WiFi module: http://www.ebay.com/itm/191542840815
• 2x4 right angle header: http://www.ebay.com/itm/271560109634
• Arduino 12V power supply: http://www.ebay.com/itm/321901850965
• Soil sensors: http://www.ebay.com/itm/131623541271
• DuPont female connectors: http://www.ebay.com/itm/171199868032
• 4 channel optocoupler relay: http://www.ebay.com/itm/381427778406

Once I have the moisture sensor software itself working, I can easily drop the sensor count to 3 and use the extra relay channel to test the power connections.

Now comes the hard part: waiting for all the bits and pieces to arrive!