Mini-split systems are comprised of an indoor evaporator unit and an outdoor condenser.
To connect the indoor and outdoor units, you will need a line set. This is an insulated copper refrigeration tube that has a liquid line to transport refrigerant from the condenser to the evaporator and a suction line to bring refrigerant from the evaporator to the condenser.
Some accessories you may need for basic installations include: equipment mounting pads, wall brackets, stands, disconnect boxes, line voltage monitors, and more.
A condensate pump may be needed when the mini-split is installed on an interior wall. One of these usually comes with a ceiling-mounted unit, but a more powerful one may be required for longer runs.
Conduit kits, also known as Whips, can be useful for wiring of the outdoor units, as well.
You will need communication wire to power and link the indoor units to the outdoor units. Check with local code and unit specs for specific wire size requirements.
Mini-splits come with a remote control, but a wall-mounted thermostat with more programming features can be useful in situations where a remote could get lost.
The efficiency of mini-splits is measured in SEER, Seasonal Energy Efficiency Ratio. The greater the SEER rating, the greater the efficiency.
Mini-split condensers and evaporators are designed as matching pairs. Mixing and matching units is not encouraged, as it can impact the efficiency of the system and there is no way of guaranteeing compatibility.