Looks to be pretty standard. Float switches aren't complicated devices. As long as the relay is of decent quality, you won't have any problems. I'm setting up an auto topoff using my Reefkeeper Lite. Since the Reefkeeper package already came with relays in the powerstrips, the float switch itself is cheaper. For the pump, I'm using the BRS aqualifter.
As for automatic water changes, you can make one yourself. You'd basically build a second auto-topoff system filled with saltwater rather than fresh. Install a third pump in your sump to remove tank water and plumb it to a nearby drain. As the water level of the sump decreases, the saltwater topoff will replenish the tank with clean saltwater. Keep the tank circulating and position the lines such that you're not draining the clean saltwater that you just put in.
If you want it completely automated, you'll need a controller. I'm not implementing an auto water changer on my tank so I haven't explored this in detail, but your controller will need to be able to turn off the freshwater topoff, turn on the saltwater topoff, and then turn on the drain pump for a desired period of time at the desired time/day of the week. After the timer expires, it will need to keep the saltwater topoff on for a short period to ensure that it catches up, then finally turn off the saltwater topoff and turn back on the freshwater topoff. I think the Reefkeeper, neptune, and similar systems can do this, but perhaps someone where more experience with these units can confirm.
