Sorry to chime in late. @Rob, to briefly answer your questions:
1) As Alan said, most residential sprinkler valves (particularly underground sprinkler system) are 24VAC type. Latching type is more common in battery powered controllers as it’s more power efficient. The simplest way to tell them apart is: 24VAC valves have two wires that have the same color; latching valve usually have a red wire and a black wire (since it’s polarized). The only product that we sell that can work with latching solenoids is the OSBee Arduino Shield:
http://rayshobby.net/cart/osbeeshield/
the video has a good explanation of 24VAC vs. latching valves.
2) AC adapters are usually unregulated. 24VAC means the output voltage is 24V when it’s at the rated current draw (say 500mA or 750mA). At no load or light load, the output voltage is typically higher than 24.
3) It’s hard to tell which RPi is better, I would think the newer models (like A+/B+) are better, but I don’t have any experimental data myself.
4) 24VAC valves (since they are non-latching) will simply turn off when power is gone — they require power to remain activated.
5) The following wiki page has instructions on watchdog timer;
http://rayshobby.net/mediawiki/index.php/Set_Up_RPi,_RTC,_WiFi,_Data_Log#Hardware_watchdog
If you use the pre-configured SD card image, I believe it has configured watchdog timer.