You'll need the low voltage sprinkler valve. There are several on the market, available at Home Depot, Lowes or any decent hardware store or sprinkler specialty shop. Probably least expensive at Depot or Lowes, and should be around $20.00. The one I have is made by a company called "Water Master" and would screw conveniently right on to the hose bib you posted above. There is a commonly available hose bib to 1/4" RO/DI hose adapter that may have come with your RO/DI, if not, Depot sells adapters to get you to the right size.
You'll also need a doorbell transformer available also at any home center or hardware store. The toughest thing to get is the float switch from Grainger. They tend to not want to do business with anyone that doesn't have a business account with them, but I have walked in off the street and bought stuff, you just have to be nice to them
Here's the switch:
http://www.grainger.com/Grainger/MADISON-Switch-2A554
You'll want to build a PVC capsule for the switch to live in so that critters can't mess with it (if a snail were to sit on that switch, it would remain in the "on" position until the snail left. That could be MANY gallons of RO/DI water added to your system causing overflows, and salinity dillution).
Here's where I got the idea/plan for the switch protector:
http://www.hawkfish.org/snailman/diytofloat.htm.
Wiring is pretty simple. The sprinkler valve is grounded to the cold water pipe, and the transformer is connected to 120V house current. The low voltage from the transformer goes to the float switch, and then the switched leg goes to the sprinkler valve. You set the switch in your sump at the level you want to maintain, and when the water level drops, the switch sends current to the sprinkler valve which opens, and feeds your RO/DI with "dirty" water to purify.
