For the autotopoff, I just took a container (5 gallon in my 15's case) elevated it to above the level of the sump, ran a line from the container to a Kent float valve, and let gravity feed the water from the container to the mechanical float switch. Works great, cost me $17 to make.
Don't use a float valve right from your RO, if the float switch failes you could have 125GPD into your sump driving your salinity to 0 perty fast. If it fails and you are using a 5 gallon bucket, then you have a little mess and a little lower salinity, but things are still alive.
Great progress, good luck!
Whiskey