To do water changes, you will absolutely have to have a vessel to hold the new saltwater, as it needs to age before being used... it is caustic when fresh.
The exact setup would depend on what you have to work with. If you have a free bulkhead low on your sump, you could plumb a drain to that, but you need it to gravity feed down, to a basement for disposal or something like that. If it has to go uphill it will have to be pump-driven. You would also need a pump in the vessel, to pump the new water out of that and back into your system. You would basically drain (or run the drain pump) to draw down the sump, then run the other pump to replace the water.
To do auto top off, all you need is an RO/DI filter and a commercially available ATO unit (the JBJ ATO is very popular). If you do this, I would highly recommend making redundant safety part of your plan. For example, the JBJ has two float switches. One signals the unit to turn on when the level drops, and to turn off when the level is back to normal. The second switch can be placed just above the water level. Its function is a backup in case the first switch malfunctions. That way, you can only add maybe 1" too much fresh water to the system before the backup switch shuts it down, and this isn't enough to hurt anything.
This will also require a holding vessel for fresh RO water, but it can be very discreet. I use a small plastic garbage can. That container can be automatically kept filled by a simple float valve which is at the connection of your filtration to this container.
You CAN use an electric solenoid inline from the RO unit, controlled via float switch, to top off without that holding container, but I personally would not recommend it since it doesn't have a backup. A bad solenoid could allow freshwater to flow into your tank all the way up until you notice it overflowing.