How to build a node sensor network with a base unit to publish to the web
Based on your component list, it appears cost is no object. At least for the prototype you're developing.

There are many wireless modules that have built in GPIO (general purpose input/output) for controlling solenoids and analog inputs for sensors. Like the RN-41 for Bluetooth or the RN171 for WiFi. Those would work well as nodes and would save the necessity for an Arduino for each node. I'm not sure on their mesh networking ability, but it's unlikely. Zigbee will give you that ability, I think they have a decent pre-built solution for that actually, but maybe with the range of WiFi you wouldn't need it.

Conceptually you have the right idea. It's difficulty is hard to say. I think it wouldn't be too much trouble for someone with very decent embedded programming skills; the modules are going to handle the hardware difficulties. The only circuit you'd probably build is the activator for the solenoid, of which I'm sure you could find a lot of examples on this site and the wider internet. I would estimate this project being 90% programming. 

Handing it off to someone else will work if you try to limit the options as much as possible. If you try to include things like an adjustable threshold for activating the solenoid then you're going to have provide software, perhaps a GUI, to configure your board or the customer is going to need the tool chain and programming skills.